The Royals also placed Ryan Yarbrough on the 60-day injured list and fellow left-hander Amir Garrett on the family medical emergency list. Garrett can be on the list for a minimum of three days and a maximum of seven.
Mayers, 31, last pitched in the majors in 2022 with the Los Angeles Angels, posting a 1-1 record with a 5.68 ERA in 24 appearances (three starts). He owns an 11-9 record with a 5.10 ERA in 198 career games (six starts) with the St. Louis Cardinals and Angels.
Yarbrough, 31, initially was placed on the 15-day injured list on May 8 after he was hit in the face by a line drive in the previous day's game against the Oakland Athletics.
Yarbrough is 1-4 with a 6.15 ERA in 10 appearances (three starts) with the Royals this season. He spent his first five major league seasons with the Tampa Bay Rays and is 41-35 in 137 appearances (62 starts) with a 4.41 ERA.
Garrett, 31, has recorded a 3.57 ERA without a decision in 19 relief appearances this season. He is 13-18 with a 5.00 ERA in 314 career appearances (14 starts) with the Cincinnati Reds and Royals.