Exhibit 23

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in Registration Statement Nos. 333-69776, 333-176085, and 333-229776 on Form S-8 of our report dated June 25, 2021, relating to the financial statements and financial statement schedules of the Marsh & McLennan Companies 401(k) Savings & Investment Plan appearing in this Annual Report on Form 11-K of the Marsh & McLennan Companies 401(k) Savings & Investment Plan for the year ended December 31, 2020.

/s/ Deloitte & Touche LLP

New York, New York

June 25, 2021