On January 30, 2020, the Tigers signed Mercer to a minor-league contract with an invitation to spring training. Mercer was released by the Tigers prior to the 2020 season in March; he re-signed with the Tigers on a minor league pact shortly after his release. Mercer made the Opening Day roster for the Tigers in 2020. On August 6, Mercer was outrighted off of the 40-man roster and he elected free agency.
On August 17, 2020, Mercer signed a minor league contract with the New York Yankees. The Yankees promoted Mercer to the major leagues on August 28. On September 1, Mercer was designated for assignment by the Yankees.