The example and the expenses in the table above should not be considered a representation of our future expenses, and actual expenses may be greater or less than those shown. While the example assumes, as required by the SEC, a 5.0% annual return, our performance will vary and may result in a return greater or less than 5.0%. In addition, while the example assumes reinvestment of all dividends at net asset value, participants in our dividend reinvestment plan will receive a number of shares of our common stock, determined by dividing the total dollar amount of the dividend payable to a participant by (i) the market price per share of our common stock at the close of trading on the dividend payment date in the event that we use newly issued shares to satisfy the share requirements of the divided reinvestment plan or (ii) the average purchase price of all shares of common stock purchased by the administrator of the dividend reinvestment plan in the event that shares are purchased in the open market to satisfy the share requirements of the dividend reinvestment plan, which may be at, above or below net asset value. See "Dividend Reinvestment Plan" in the accompanying prospectus for additional information regarding our dividend reinvestment plan.

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SUPPLEMENTAL RISK FACTORS

        Investing in our common stock involves a number of significant risks. In addition to the other information contained in this prospectus supplement and the accompanying prospectus, you should consider carefully the following supplementary risk factors together with the risk factors beginning on page 13 of the accompanying prospectus before making an investment in our common stock. The risks set out below and in the accompanying prospectus are not the only risks we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us might also impair our operations and performance. If any of the events described herein or in the accompanying prospectus occur, our business, financial condition and results of operations could be materially and adversely affected. In such case, our net asset value and the trading price of our common stock could decline, and you may lose all or part of your investment.

Risks Relating to Economic Conditions

The current state of the economy and financial markets increases the likelihood of adverse effects on our financial position and results of operations. Continued economic adversity could impair our portfolio companies' financial positions and operating results and affect the industries in which we invest, which could, in turn, harm our operating results.

        The broader economic fundamentals of the United States economy remain uncertain. Unemployment levels remain elevated and other economic fundamentals remain depressed. In the event that the United States economic performance contracts, it is likely that the financial results of small- to mid-sized companies, like those in which we invest, could experience deterioration or limited growth, which could ultimately lead to difficulty in meeting their debt service requirements and an increase in defaults. Consequently, we can provide no assurance that the performance of certain of our portfolio companies will not be negatively impacted by economic or other conditions, which could also have a negative impact on our future results.

        Although we have been able to secure access to additional liquidity, including our $100 million credit facility, periodic follow on equity offerings, and the increase in available leverage through the SBIC program as part of the American Recovery and Reinvestment Act of 2009 enacted in February 2009 (the "Stimulus Bill"), the potential for volatility in the debt and in the equity capital markets provides no assurance that debt or equity capital will be available to us in the future on favorable terms, or at all.

Risks Relating to Our Business and Structure

Because we borrow money, the potential for gain or loss on amounts invested in us is magnified and may increase the risk of investing in us.

        Borrowings, also known as leverage, magnify the potential for gain or loss on invested equity capital. As we use leverage to partially finance our investments, you will experience increased risks of investing in our common stock. We, through the Funds, issue debt securities guaranteed by the SBA and sold in the capital markets. As a result of its guarantee of the debt securities, the SBA has fixed dollar claims on the assets of the Funds that are superior to the claims of our common stockholders. We may also borrow from banks and other lenders, including under the $100 million credit facility we entered into during 2010 and expanded during January 2011. See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Capital Resources" in this prospectus supplement for a discussion regarding our credit facility. If the value of our assets increases, then leveraging would cause the net asset value attributable to our common stock to increase more sharply than it would have had we not leveraged. Conversely, if the value of our assets decreases, leveraging would cause net asset value to decline more sharply than it otherwise would have had we not leveraged our business. Similarly, any increase in our income in excess of interest payable on the borrowed funds would cause our net investment income to increase more than it would without the leverage, while any

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decrease in our income would cause net investment income to decline more sharply than it would have had we not borrowed. Such a decline could negatively affect our ability to pay common stock dividends. Leverage is generally considered a speculative investment technique.

        As of December 31, 2010, we, through the Funds, had $180 million of outstanding indebtedness guaranteed by the SBA, which had a weighted average annualized interest cost of approximately 5.2% (exclusive of deferred financing costs). The debentures guaranteed by the SBA have a maturity of ten years and require semi-annual payments of interest. We will need to generate sufficient cash flow to make required interest payments on the debentures. If we are unable to meet the financial obligations under the debentures, the SBA, as a creditor, will have a superior claim to the assets of the Funds over our stockholders in the event we liquidate or the SBA exercises its remedies under such debentures as the result of a default by us. In addition, as of December 31, 2010, we had $39 million outstanding under the Credit Facility. Borrowings under the Credit Facility bear interest, subject to our election, on a per annum basis equal to (i) the applicable LIBOR rate plus 2.50% or (ii) the applicable base rate plus 1.50%. Main Street pays unused commitment fees of 0.375% per annum on the average unused lender commitments under the Credit Facility. If we are unable to meet the financial obligations under the Credit Facility, the Credit Facility lending group will have a superior claim to the assets of MSCC and its subsidiaries (excluding the assets of the Funds) over our stockholders in the event we liquidate or the lending group exercises its remedies under the Credit Facility as the result of a default by us.

        Illustration.    The following table illustrates the effect of leverage on returns from an investment in our common stock assuming various annual returns, net of expenses. The calculations in the table below are hypothetical and actual returns may be higher or lower than those appearing below.

Assumed Return on Our Portfolio(1)
(net of expenses)

        Our ability to achieve our investment objective may depend in part on our ability to access additional leverage on favorable terms by issuing debentures guaranteed by the SBA, through the Funds, or by borrowing from banks, insurance companies or other commercial lenders, and there can be no assurance that such additional leverage can in fact be achieved.

Risks Related to Our Investments

Our investments in portfolio companies involve higher levels of risk, and we could lose all or part of our investment.

        Investing in our portfolio companies involves a number of significant risks. Among other things, these companies:

•

may have limited financial resources and may be unable to meet their obligations under their debt instruments that we hold, which may be accompanied by a deterioration in the value of any collateral and a reduction in the likelihood of us realizing any guarantees from subsidiaries or affiliates of our portfolio companies that we may have obtained in connection with our investment, as well as a corresponding decrease in the value of the equity components of our investments;

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•

may have shorter operating histories, narrower product lines, smaller market shares and/or significant customer concentrations than larger businesses, which tend to render them more vulnerable to competitors' actions and market conditions, as well as general economic downturns;



•

are more likely to depend on the management talents and efforts of a small group of persons; therefore, the death, disability, resignation, termination, or significant under-performance of one or more of these persons could have a material adverse impact on our portfolio company and, in turn, on us;



•

generally have less predictable operating results, may from time to time be parties to litigation, may be engaged in rapidly changing businesses with products subject to a substantial risk of obsolescence, and may require substantial additional capital to support their operations, finance expansion or maintain their competitive position; and



•

generally have less publicly available information about their businesses, operations and financial condition. We are required to rely on the ability of our management team and investment professionals to obtain adequate information to evaluate the potential returns from investing in these companies. If we are unable to uncover all material information about these companies, we may not make a fully informed investment decision, and may lose all or part of our investment.

        In addition, in the course of providing significant managerial assistance to certain of our portfolio companies, certain of our officers and directors may serve as directors on the boards of such companies. To the extent that litigation arises out of our investments in these companies, our officers and directors may be named as defendants in such litigation, which could result in an expenditure of funds (through our indemnification of such officers and directors) and the diversion of management time and resources.

Our marketable securities and idle funds investments are subject to risks including risks similar to our portfolio investments.

        Marketable securities and idle funds investments can include, among other things, secured and unsecured debt investments, independently rated debt investments and diversified bond funds. Many of these investments in debt obligations are, or would be if rated, below investment grade quality. Indebtedness of below investment grade quality is regarded as having predominantly speculative characteristics with respect to the issuer's capacity to pay interest and repay principal, similar to our portfolio investments in the lower middle market. (See "—Our investments in portfolio companies involve higher levels of risk, and we could lose all or part of our investment" in this prospectus supplement.) Many of these marketable securities and idle funds investments are purchased through over the counter or other markets and are therefore liquid at the time of purchase but may subsequently become illiquid due to events relating to the issuer of the securities, market events, economic conditions or investor perceptions. See "Risk Factors—Risks Related to Our Investments—The lack of liquidity in our investments may adversely affect our business" in the accompanying prospectus for a description of risks related to holding illiquid investments. In addition, domestic and foreign markets are complex and interrelated, so that events in one sector of the world markets or economy, or in one geographical region, can reverberate and have materially negative consequences for other market, economic or regional sectors in a manner that may not be foreseen and which may materially affect the market price of our marketable securities and idle funds investments. Other risks that our portfolio investments are subject to are also applicable to these marketable securities and idle funds investments. See "Risk Factors—Risks Related to Our Investments" in the accompanying prospectus and "—Risks Related to Our Investments" in this prospectus supplement for risks affecting our portfolio investments.

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USE OF PROCEEDS

        The net proceeds from the sale of the            shares of common stock in this offering are $            , and $            if the underwriter's over-allotment option is exercised in full, after deducting the underwriting discount and estimated offering expenses of approximately $200,000 payable by us.

        We intend to use the net proceeds from this offering to repay outstanding debt borrowed under our $100 million credit facility. We intend to use any remaining net proceeds from this offering to make investments in accordance with our investment objective and strategies described in this prospectus supplement and the accompanying prospectus, pay our operating expenses and other cash obligations, and for general corporate purposes. Pending such uses, we currently intend to invest the net proceeds of this offering in marketable securities and idle funds investments, which may include investments in secured intermediate term bank debt, rated debt securities and other income producing investments, consistent with our BDC election and our election to be taxed as a RIC. See "Regulation—Regulation as a Business Development Company" in the accompanying prospectus.

        At March 18, 2011, we had approximately $65.0 million outstanding under our $100 million credit facility. Our credit facility matures on September 20, 2013, unless extended, and bears interest, at our election, on a per annum basis equal to (i) the applicable LIBOR rate plus 2.50% or (ii) the applicable base rate plus 1.50%. Amounts repaid under our $100 million credit facility will remain available for future borrowings.

        Affiliates of Morgan Keegan & Company, Inc. and BB&T Capital Markets, each an underwriter in this offering, act as lenders and/or agents under our $100 million credit facility. As described above, we intend to use net proceeds of this offering to repay the outstanding indebtedness under this credit facility, and those affiliates therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See "Underwriting—Conflicts of Interest" below.

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CAPITALIZATION

        The following table sets forth our capitalization:

•

on an actual basis as of December 31, 2010; and



•

on an as-adjusted basis giving effect to the sale of            shares of our common stock in this offering at the public offering price of $            per share, less estimated underwriting discounts and offering expenses payable by us.

        This table should be read in conjunction with "Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Audited Financial Statements" in this prospectus supplement.

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PRICE RANGE OF COMMON STOCK AND DISTRIBUTIONS

        Our common stock is traded on the New York Stock Exchange ("NYSE") under the symbol "MAIN." Prior to October 14, 2010, our common stock was traded on the NASDAQ Global Select Market under the same symbol "MAIN." Our common stock began trading on the NASDAQ Global Select Market on October 5, 2007. Prior to that date, there was no established public trading market for our common stock.

        The following table sets forth, for each fiscal quarter since our common stock began trading, the range of high and low closing prices of our common stock as reported on the NYSE and on the NASDAQ Global Select Market, as applicable, and the sales price as a percentage of our net asset value per share ("NAV").

        On March 18, 2011, the last sale price of our common stock on the NYSE was $19.19 per share, and there were approximately 216 holders of record of the common stock which did not include shareholders for whom shares are held in "nominee" or "street name."

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        Shares of BDCs may trade at a market price that is less than the value of the net assets attributable to those shares. The possibility that our shares of common stock will trade at a discount from net asset value or at premiums that are unsustainable over the long term are separate and distinct from the risk that our net asset value will decrease. It is not possible to predict whether the common stock offered hereby will trade at, above, or below net asset value. Since our IPO in October 2007, our shares of common stock have traded at prices both less than and exceeding our net asset value.

        From our IPO through the third quarter of 2008, we paid quarterly dividends, but in the fourth quarter of 2008 we began paying, and we intend to continue paying, monthly dividends to our stockholders. Our monthly dividends, if any, will be determined by our Board of Directors on a quarterly basis.

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        The following table summarizes our dividends declared to date:

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        To obtain and maintain RIC tax treatment, we must, among other things, distribute at least 90% of our net ordinary income and realized net short-term capital gains in excess of realized net long-term capital losses, if any. We will be subject to a 4% nondeductible federal excise tax on certain undistributed taxable income unless we distribute in a timely manner an amount at least equal to the sum of (1) 98% of our net ordinary income for each calendar year, (2) 98.2% of our capital gain net income for the one-year period ending December 31 in that calendar year and (3) any income recognized, but not distributed, in preceding years (the "Excise Tax Avoidance Requirement"). Dividends declared and paid by us in a year will generally differ from taxable income for that year, as such dividends may include the distribution of current year taxable income, less amounts carried over into the following year, and the distribution of prior year taxable income carried over into and distributed in the current year. For amounts we carry over into the following year, we will be required to pay the 4% excise tax described above. We may retain for investment some or all of our net capital gains (i.e., realized net long-term capital gains in excess of realized net short-term capital losses) and treat such amounts as deemed distributions to our stockholders. If we do this, our stockholders will be treated as if they had received actual distributions of the capital gains we retained and then reinvested the net after-tax proceeds in our common stock. In general, our stockholders also would be eligible to claim a tax credit (or, in certain circumstances, a tax refund) equal to their allocable shares of the tax we paid on the capital gains deemed distributed to them. We can offer no assurance that we will achieve results that will permit the payment of any cash distributions and, if we issue senior securities, we may be prohibited from making distributions if doing so causes us to fail to maintain the asset coverage ratios stipulated by the 1940 Act or if distributions are limited by the terms of any of our borrowings.

        Pursuant to a revenue procedure issued by the Internal Revenue Service, or the IRS, the IRS has indicated that it will treat distributions from certain publicly traded RICs (including business development companies) that are paid part in cash and part in stock as dividends that would satisfy the RIC's annual distribution requirements. In order to qualify for such treatment, the revenue procedure requires that at least 10% of the total distribution be paid in cash and that each stockholder have a right to elect to receive its entire distribution in cash. If too many stockholders elect to receive cash, each stockholder electing to receive cash must receive a proportionate share of the cash to be distributed (although no stockholder electing to receive cash may receive less than 10% of such stockholder's distribution in cash). This revenue procedure applies to distributions declared on or before December 31, 2012, with respect to taxable years ended on or before December 31, 2011.

        Where the IRS revenue procedure is not currently applicable, the IRS has also issued private letter rulings on cash and stock dividends paid by RICs and real estate investment trusts using a 20% cash standard (and, more recently, the 10% cash standard of the above-referenced IRS revenue procedure) if certain requirements are satisfied.

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SELECTED FINANCIAL DATA

        The selected financial data below reflects the combined operations of MSMF and MSMF GP for the years ended December 31, 2006 and the consolidated operations of Main Street and its subsidiaries for the years ended December 31, 2007, 2008, 2009, and 2010. The selected financial data at December 31, 2006, 2007, 2008, 2009, and 2010 and for the years ended December 31, 2006, 2007, 2008, 2009, and 2010, have been derived from combined/consolidated financial statements that have been audited by Grant Thornton LLP, an independent registered public accounting firm. You should read this selected financial data in conjunction with our "Management's Discussion and Analysis of Financial Condition and Results of Operations" and the financial statements and related notes included in this prospectus.

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MANAGEMENT'S DISCUSSION AND ANALYSIS OF
FINANCIAL CONDITION AND RESULTS OF OPERATIONS

        The following discussion should be read in conjunction with our consolidated financial statements contained elsewhere in this prospectus supplement.

        Statements we make in the following discussion which express a belief, expectation or intention, as well as those that are not historical fact, are forward-looking statements that are subject to risks, uncertainties and assumptions. Our actual results, performance or achievements, or industry results, could differ materially from those we express in the following discussion as a result of a variety of factors, including the risks and uncertainties we have referred to under the headings "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors" in the accompanying prospectus and "Supplemental Risk Factors" in this prospectus supplement.

ORGANIZATION

        Main Street Capital Corporation ("MSCC") was formed on March 9, 2007 for the purpose of (i) acquiring 100% of the equity interests of Main Street Mezzanine Fund, LP ("MSMF") and its general partner, Main Street Mezzanine Management, LLC ("MSMF GP"), (ii) acquiring 100% of the equity interests of Main Street Capital Partners, LLC (the "Investment Manager"), (iii) raising capital in an initial public offering, which was completed in October 2007 (the "IPO"), and (iv) thereafter operating as an internally managed business development company ("BDC") under the Investment Company Act of 1940, as amended (the "1940 Act"). MSMF is licensed as a Small Business Investment Company ("SBIC") by the United States Small Business Administration ("SBA") and the Investment Manager acts as MSMF's manager and investment adviser. Because the Investment Manager, which employs all of the executive officers and other employees of MSCC, is wholly owned by us, we do not pay any external investment advisory fees, but instead we incur the net operating costs associated with employing investment and portfolio management professionals through the Investment Manager. The IPO and related transactions discussed above were consummated in October 2007 and are collectively termed the "Formation Transactions."

        On January 7, 2010, MSCC consummated transactions (the "Exchange Offer") to exchange 1,239,695 shares of its common stock for approximately 88% of the total dollar value of the limited partner interests in Main Street Capital II, LP ("MSC II" and, together with MSMF, the "Funds"). Pursuant to the terms of the Exchange Offer, 100% of the membership interests in the general partner of MSC II, Main Street Capital II GP, LLC ("MSC II GP"), were also transferred to MSCC for no consideration. MSC II commenced operations in January 2006, is an investment fund that operates as an SBIC and is also managed by the Investment Manager. The Exchange Offer and related transactions, including the transfer of the MSC II GP interests, are collectively termed the "Exchange Offer Transactions" (see Note J to the consolidated financial statements included in this prospectus supplement). As of December 31, 2010, an approximately 12% minority ownership in the total dollar value of the MSC II limited partnership interests remained outstanding, including approximately 5% owned by affiliates of MSCC.

        MSCC has direct or indirect subsidiaries that have elected to be taxable entities (the "Taxable Subsidiaries"). The primary purpose of these entities is to hold certain investments that generate "pass through" income for tax purposes. The Taxable Subsidiaries are each taxed at their normal corporate tax rates based on their taxable income.

        Unless otherwise noted or the context otherwise indicates, the terms "we," "us," "our" and "Main Street" refer to MSCC and its subsidiaries, including MSMF, MSC II, and the Taxable Subsidiaries.

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OVERVIEW

        We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies, which we generally define as companies with annual revenues between $10 million and $100 million that operate in diverse industries. We invest primarily in secured debt instruments, equity investments, warrants and other securities of LMM companies based in the United States. Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company. Our LMM portfolio investments generally range in size from $3 million to $20 million.

        We seek to fill the current financing gap for LMM businesses, which, historically, have had more limited access to financing from commercial banks and other traditional sources. The underserved nature of the lower middle market creates the opportunity for us to meet the financing needs of LMM companies while also negotiating favorable transaction terms and equity participations. Our ability to invest across a company's capital structure, from senior secured loans to equity securities, allows us to offer portfolio companies a comprehensive suite of financing solutions, or "one stop" financing. Providing customized, "one stop" financing solutions has become even more relevant to our LMM portfolio companies in the current investing environment. We generally seek to partner directly with entrepreneurs, management teams and business owners in making our investments. We believe that our LMM investment strategy has a lower correlation to the broader debt and equity markets.

        In addition to our primary investment strategy of investing in LMM companies, we opportunistically pursue investments in privately placed debt securities. Our private placement investment portfolio primarily consists of direct or secondary private placements of interest bearing debt securities in companies that are generally larger in size than the LMM companies included in our investment portfolio. As of December 31, 2010, we had privately placed portfolio investments in 16 companies collectively totaling approximately $67.1 million in fair value with a total cost basis of approximately $65.6 million. The weighted average revenues for the 16 privately placed portfolio investments was approximately $352 million. All of our privately placed portfolio investments were in the form of debt investments and 71% of such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average effective yield on our privately placed portfolio debt investments was approximately 12.5% as of December 31, 2010.

        Our portfolio investments are generally made through MSCC and the Funds. MSCC and the Funds share the same investment strategies and criteria, although they are subject to different regulatory regimes. An investor's return in MSCC will depend, in part, on the Funds' investment returns as MSMF is a wholly owned subsidiary of MSCC and MSC II is a majority owned subsidiary of MSCC.

        The level of new portfolio investment activity will fluctuate from period to period based upon our view of the current economic fundamentals, our ability to identify new investment opportunities that meet our investment criteria, and our ability to consummate the identified opportunities. The level of new investment activity, and associated interest and fee income, will directly impact future investment income. In addition, the level of dividends paid by portfolio companies and the portion of our portfolio debt investments on non-accrual status will directly impact future investment income. While we intend to grow our portfolio and our investment income over the long-term, our growth and our operating results may be more limited during depressed economic periods. However, we intend to appropriately manage our cost structure and liquidity position based on applicable economic conditions and our investment outlook. The level of realized gains or losses and unrealized appreciation or depreciation will also fluctuate depending upon portfolio activity and the performance of our individual portfolio

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companies. The changes in realized gains and losses and unrealized appreciation or depreciation could have a material impact on our operating results.

        For the year ended December 31, 2010, we paid dividends on a monthly basis totaling $1.50 per share, or $23.9 million. We generated undistributed taxable income (or "spillover income") of approximately $1.6 million, or $0.09 per share, during 2010 that will be carried forward toward distributions paid in 2011. In December 2010, we declared monthly dividends for the first quarter of 2011 totaling $0.375 per share. In March 2011, we declared monthly dividends for the second quarter of 2011 totaling $0.39 per share. Including the dividends declared for the first and second quarters of 2011, we will have paid approximately $5.52 per share in cumulative dividends since our October 2007 initial public offering.

        At December 31, 2010, we had $91.1 million in cash and cash equivalents, marketable securities, and idle funds investments. In August 2010, we completed a follow-on public stock offering in which we sold 3,220,000 shares of common stock, including the underwriters' exercise of the over-allotment option, at a price to the public of $15.00 per share (or approximately 123% of the then latest reported Net Asset Value per share), resulting in total net proceeds of approximately $45.8 million, after deducting underwriters' commissions and offering costs. In January 2010, we completed a follow-on public stock offering in which we sold 2,875,000 shares of common stock, including the underwriters' exercise of the over-allotment option, at a price to the public of $14.75 per share (or approximately 123% of the then latest reported Net Asset Value per share), resulting in total net proceeds of approximately $40.1 million, after deducting underwriters' commissions and offering costs.

CRITICAL ACCOUNTING POLICIES

Basis of Presentation

        Our financial statements are prepared in accordance with U.S. generally accepted accounting principles ("U.S. GAAP"). For the year ended December 31, 2010, our consolidated financial statements include the accounts of MSCC and its consolidated subsidiaries, including MSMF and MSC II. For the year ended 2009, our consolidated financial statements include the accounts of MSCC and its consolidated subsidiaries, including MSMF. Portfolio investments, as used herein, refers to all of our portfolio investments in LMM companies, private placement portfolio investments, and the Investment Manager and excludes all "Marketable securities and idle funds investments." Private placement portfolio investments include investments made through direct or secondary purchases of interest-bearing debt securities in companies that are generally larger in size than the LMM companies included as part of our portfolio investments. "Marketable securities and idle funds investments" are classified as financial instruments and are reported separately on our Consolidated Balance Sheets and Consolidated Schedule of Investments due to the nature of such investments. Our results of operations and cash flows for the years ended December 31, 2010, 2009, and 2008, and financial position as of December 31, 2010 and 2009, are presented on a consolidated basis. The effects of all intercompany transactions between Main Street and its consolidated subsidiaries have been eliminated in consolidation. Certain reclassifications have been made to prior period balances to conform with the current financial statement presentation, including the reclassification of private placement portfolio investments which were formerly classified as "Marketable securities and idle funds investments" and are now classified as portfolio investments in the "Non-Control/Non-Affiliate investments" category due to our current intent to hold such investments until their maturity and the fact that their terms adhere more to our portfolio investment strategy.

        Under the investment company rules and regulations pursuant to Article 6 of Regulation S-X and the Audit and Accounting Guide for Investment Companies issued by the American Institute of Certified Public Accountants (the "AICPA Guide"), we are precluded from consolidating portfolio company investments, including those in which we have a controlling interest, unless the portfolio

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company is another investment company. An exception to this general principle in the AICPA Guide occurs if we own a controlled operating company that provides all or substantially all of its services directly to us, or to an investment company of ours. None of the investments made by us qualify for this exception. Therefore, our portfolio investments are carried on the balance sheet at fair value, as discussed further in Note B to our consolidated financial statements, with any adjustments to fair value recognized as "Net Change in Unrealized Appreciation (Depreciation)" on our Statement of Operations until the investment is exited, resulting in any gain or loss on exit being recognized as a "Net Realized Gain (Loss) from Investments."

Portfolio Investment Valuation

        The most significant estimate inherent in the preparation of our consolidated financial statements is the valuation of our portfolio investments and the related amounts of unrealized appreciation and depreciation. As of December 31, 2010 and 2009, approximately 78% and 80%, respectively, of our total assets represented investments in portfolio companies valued at fair value (including the investment in the Investment Manager). We are required to report our investments at fair value. We adopted the provisions of the Accounting Standards Codification ("Codification" or "ASC") 820, Fair Value Measurements and Disclosures in the first quarter of 2008. ASC 820 defines fair value, establishes a framework for measuring fair value, establishes a fair value hierarchy based on the quality of inputs used to measure fair value, and enhances disclosure requirements for fair value measurements.

        Our portfolio strategy calls for us to invest primarily in illiquid securities issued by private, LMM companies. These LMM portfolio investments may be subject to restrictions on resale and will generally have no established trading market. We determine in good faith the fair value of our LMM portfolio investments pursuant to a valuation policy in accordance with ASC 820 and a valuation process approved by our Board of Directors and in accordance with the 1940 Act. We review external events, including private mergers, sales and acquisitions involving comparable companies, and include these events in the valuation process. Our valuation policy and process are intended to provide a consistent basis for determining the fair value of the portfolio.

        For valuation purposes, control investments are composed of equity and debt securities for which we have a controlling interest in the portfolio company or have the ability to nominate a majority of the portfolio company's board of directors. Market quotations are generally not readily available for our control investments. As a result, we determine the fair value of control investments using a combination of market and income approaches. Under the market approach, we will typically use the enterprise value methodology to determine the fair value of these investments. The enterprise value is the fair value at which an enterprise could be sold in a transaction between two willing parties, other than through a forced or liquidation sale. Typically, private companies are bought and sold based on multiples of earnings before interest, taxes, depreciation and amortization, or EBITDA, cash flows, net income, revenues, or in limited cases, book value. There is no single methodology for estimating enterprise value. For any one portfolio company, enterprise value is generally described as a range of values from which a single estimate of enterprise value is derived. In estimating the enterprise value of a portfolio company, we analyze various factors, including the portfolio company's historical and projected financial results. We allocate the enterprise value to investments in order of the legal priority of the investments. We will also use the income approach to determine the fair value of these securities, based on projections of the discounted future free cash flows that the portfolio company or the debt security will likely generate. The valuation approaches for our control investments estimate the value of the investment if we were to sell, or exit, the investment, assuming the highest and best use of the investment by market participants. In addition, these valuation approaches consider the value associated with our ability to control the capital structure of the portfolio company, as well as the timing of a potential exit.

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        For valuation purposes, non-control LMM portfolio investments are composed of debt and equity securities for which we do not have a controlling interest in the portfolio company, or the ability to nominate a majority of the portfolio company's board of directors. Market quotations for non-control LMM investments are generally not readily available. For our non-control LMM portfolio investments, we use a combination of the market and income approaches to value our equity investments and the income approach to value our debt instruments. For non-control LMM debt investments, we determine the fair value primarily using a yield approach that analyzes the discounted cash flows of interest and principal for the debt security, as set forth in the associated loan agreements, as well as the financial position and credit risk of each of these portfolio investments. Our estimate of the expected repayment date of a LMM debt security is generally the legal maturity date of the instrument, as we generally intend to hold our loans to maturity. The yield analysis considers changes in leverage levels, credit quality, portfolio company performance and other factors. We will use the value determined by the yield analysis as the fair value for that security; however, because of our general intent to hold our loans to maturity, the fair value will not exceed the face amount of the LMM debt security. A change in the assumptions that we use to estimate the fair value of our LMM debt securities using the yield analysis could have a material impact on the determination of fair value. If there is deterioration in credit quality or a debt security is in workout status, we may consider other factors in determining the fair value of a LMM debt security, including the value attributable to the debt security from the enterprise value of the portfolio company or the proceeds that would be received in a liquidation analysis.

        Our private placement portfolio investments primarily consist of direct or secondary purchases of interest-bearing securities in companies that are generally larger in size than the LMM companies included in our investment portfolio. For valuation purposes, all of our private placement portfolio investments are non-control investments and are composed of debt securities for which we do not have a controlling interest in the portfolio company, or the ability to nominate a majority of the portfolio company's board of directors. We primarily use observable inputs to determine the fair value of these investments through obtaining third party quotes or independent pricing.

        Due to the inherent uncertainty in the valuation process, our estimate of fair value may differ materially from the values that would have been used had a ready market for the securities existed. In addition, changes in the market environment, portfolio company performance and other events that may occur over the lives of the investments may cause the gains or losses ultimately realized on these investments to be materially different than the valuations currently assigned. We estimate the fair value of each individual investment and record changes in fair value as unrealized appreciation or depreciation.

Revenue Recognition

Interest and Dividend Income

        We record interest and dividend income on the accrual basis to the extent amounts are expected to be collected. Dividend income is recorded as dividends are declared or at the point an obligation exists for the portfolio company to make a distribution. In accordance with our valuation policy, we evaluate accrued interest and dividend income periodically for collectability. When a loan or debt security becomes 90 days or more past due, and if we otherwise do not expect the debtor to be able to service all of its debt or other obligations, we will generally place the loan or debt security on non-accrual status and cease recognizing interest income on that loan or debt security until the borrower has demonstrated the ability and intent to pay contractual amounts due. If a loan or debt security's status significantly improves regarding ability to service the debt or other obligations, or if a loan or debt security is fully impaired, sold or written off, we will remove it from non-accrual status.

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Fee Income

        We may periodically provide services, including structuring and advisory services, to our portfolio companies. For services that are separately identifiable and evidence exists to substantiate fair value, income is recognized as earned, which is generally when the investment or other applicable transaction closes. Fees received in connection with debt financing transactions for services that do not meet these criteria are treated as debt origination fees and are accreted into interest income over the life of the financing.

Payment-in-Kind ("PIK") Interest

        While not significant to the total portfolio, we hold debt and preferred equity instruments in our investment portfolio that contain payment-in-kind ("PIK") interest and cumulative dividend provisions. The PIK interest, computed at the contractual rate specified in each debt agreement, is added to the principal balance of the debt and is recorded as interest income. Thus, the actual collection of this interest may be deferred until the time of debt principal repayment. Cumulative dividends are recorded as dividend income, and any dividends in arrears are added to the balance of the preferred equity investment. The actual collection of dividends in arrears may be deferred until such time as the preferred equity is redeemed. To maintain regulated investment company ("RIC") tax treatment (as discussed below), these non-cash sources of income will need to be paid out to stockholders in the form of distributions, even though we may not have collected the PIK interest and cumulative dividends in cash. We will stop accruing PIK interest and cumulative dividends and will write off any accrued and uncollected interest and dividends in arrears when it is determined that such PIK interest and dividends in arrears are no longer collectible.

Share-Based Compensation

        We account for our share-based compensation plans using the fair value method, as prescribed by ASC 718, Compensation—Stock Compensation. Accordingly, for restricted stock awards, we measured the grant date fair value based upon the market price of our common stock on the date of the grant and will amortize this fair value to share-based compensation expense over the requisite service period or vesting term.

Income Taxes

        MSCC has elected and intends to continue to qualify for the tax treatment applicable to a RIC under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code"), and, among other things, intends to make the required distributions to its stockholders as specified therein. In order to qualify as a RIC, MSCC is required to timely distribute to its stockholders at least 90% of investment company taxable income, as defined by the Code, each year. Depending on the level of taxable income earned in a tax year, MSCC may choose to carry forward taxable income in excess of current year distributions into the next tax year and pay a 4% excise tax on such income. Any such carryover taxable income must be distributed through a dividend declared prior to filing the final tax return related to the year which generated such taxable income.

        The Taxable Subsidiaries hold certain portfolio investments of Main Street. The Taxable Subsidiaries are consolidated for U.S. GAAP reporting purposes, and the portfolio investments held by them are included in Main Street's consolidated financial statements. The Taxable Subsidiaries permit Main Street to hold equity investments in portfolio companies which are "pass through" entities for tax purposes in order to comply with the "source income" requirements contained in the RIC tax provisions. The Taxable Subsidiaries are not consolidated with Main Street for income tax purposes and may generate income tax expense, or benefit, as a result of their ownership of certain portfolio

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investments. This income tax expense, or benefit, is reflected in Main Street's Consolidated Statement of Operations.

        The Taxable Subsidiaries use the liability method in accounting for income taxes. Deferred tax assets and liabilities are recorded for temporary differences between the tax basis of assets and liabilities and their reported amounts in the financial statements, using statutory tax rates in effect for the year in which the temporary differences are expected to reverse. A valuation allowance is provided against deferred tax assets when it is more likely than not that some portion or all of the deferred tax asset will not be realized.

        Taxable income generally differs from net income for financial reporting purposes due to temporary and permanent differences in the recognition of income and expenses. Taxable income generally excludes net unrealized appreciation or depreciation, as investment gains or losses are not included in taxable income until they are realized.

LMM PORTFOLIO COMPOSITION

        LMM portfolio investments principally consist of secured debt, equity warrants and direct equity investments in privately held, LMM companies. The LMM debt investments are secured by either a first or second lien on the assets of the portfolio company, generally bear interest at fixed rates, and generally mature between five and seven years from the original investment. In most LMM portfolio companies, we also receive nominally priced equity warrants and/or make direct equity investments, usually in connection with a debt investment.

        Summaries of the composition of our LMM investment portfolio at cost and fair value as a percentage of total LMM portfolio investments are shown in the following table:

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        The following table shows the LMM portfolio composition by geographic region of the United States at cost and fair value as a percentage of total LMM portfolio investments. The geographic composition is determined by the location of the corporate headquarters of the portfolio company:

        Main Street's LMM portfolio investments are generally in companies conducting business in a variety of industries. Set forth below are tables showing the composition of Main Street's LMM portfolio investments by industry at cost and fair value as of December 31, 2010 and 2009:

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