The Royals selected Mengden's contract from Triple-A Omaha ahead of his scheduled start Thursday against the White Sox in Chicago.
Kansas City had a spot on the 40-man roster already open, so no corresponding transaction was needed to add Mengden, who is one of the Royals' two September call-ups. The 29-year-old southpaw has previously made 48 big-league starts, though his last one came in 2020 with Oakland. While pitching primarily for Omaha this season, Mengden has produced a 4.55 ERA, 1.47 WHIP and 76:50 K:BB across 91 frames. He could hold down a spot in the rotation until the Royals return Zack Greinke (forearm) from the 15-day injured list.