The Royals handed the Athletics their third straight loss.
Josh Donaldson gave Oakland a 2-1 lead in the fourth with a solo homer and that was still the score when Kansas City batted in the seventh. After retiring the first two men in the inning, McCarthy (6-4) surrendered doubles to Eric Hosmer, Cain and Getz.
The go-ahead double from Getz came at the end of an 11-pitch at-bat.
McCarthy had not lost a game since April 21.
Will Smith (4-4) got the win, allowing two runs on five hits in seven innings.
Aaron Crow worked a perfect eighth inning to protect the one-run lead and Greg Holland survived a leadoff single in the ninth to pick up his fourth save.