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( Mark One ) Annual Report Pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 For the fiscal year ended December 31 , 2020 OR Delaware ( State or other jurisdiction of incorporation or organization ) Transition Report Pursuant to Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 Commission File No. 1-5998 UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON , D.C. 20549 FORM 10 - K Title of each class Common Stock , par value $ 1.00 per share Large Accelerated Filer Non - Accelerated Filer X MARSH & MCLENNAN COMPANIES Marsh & McLennan Companies , Inc. ( Exact name of registrant as specified in its charter ) 1166 Avenue of the Americas New York , New York 10036-2774 ( Address of principal executive offices ; Zip Code ) ( 212 ) 345-5000 Registrant's telephone number , including area code Securities registered pursuant to Section 12 ( b ) of the Act : Trading symbol ( s ) MMC 36-2668272 ( I.R.S. Employer Identification No. ) Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes ý No ¨ Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes No ý Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes ý No Name of each exchange on which registered New York Stock Exchange Chicago Stock Exchange London Stock Exchange Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes ý No Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer or a smaller reporting Company . See the definitions of " large accelerated filer , " " accelerated filer " and " smaller reporting Company " in Rule 12b - 2 of the Exchange Act . ( Check one ) : Accelerated Filer Smaller Reporting Company Emerging Growth Company 000 If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant is a shell Company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes No ý Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Yes > No " As of June 30 , 2020 , the aggregate market value of the registrant's common stock held by non - affiliates of the registrant was approximately 53,818,358,381 computed by reference to the closing price of such stock as reported on the New York Stock Exchange on June 30 , 2020 . As of February 12 , 2021 , there were outstanding 508,186,561 shares of common stock , par value $ 1.00 per share , of the registrant . DOCUMENTS INCORPORATED BY REFERENCE Portions of Marsh & McLennan Companies , Inc.'s Notice of Annual Meeting and Proxy Statement for the 2021 Annual Meeting of Stockholders ( the " 2021 Proxy Statement " ) are incorporated by reference in Part III of this Form 10 - K .