Exhibit 32.1

Certification by the Chief Executive Officer

Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002

In connection with the quarterly report of Mereo BioPharma Group plc on Form 10-Q for the quarterly period ended March 31, 2024 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Denise Scots-Knight, Chief Executive Officer of the Company, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that to my knowledge:

(1) The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2) The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the registrant.

Date: May 15, 2024