EXHIBIT 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in this Registration Statement on Form S-8 of our reports dated February 20, 2020, relating to the consolidated financial statements of Marsh & McLennan Companies, Inc. (the “Company”) and the effectiveness of the Company’s internal control over financial reporting, appearing in the Annual Report on Form 10-K of Marsh & McLennan Companies, Inc. for the year ended December 31, 2019.

/s/ DELOITTE & TOUCHE LLP

New York, New York

July 31, 2020