Mills was recalled from Triple-A Omaha on Friday.
The right-hander was acquired from Seattle earlier in the week in the Carlos Santana trade, and he will now join the major-league team after just one appearance with the Royals' Triple-A affiliate. Mills pitched to a 4.15 ERA, 0.92 WHIP and 6:3 K:BB over 8.2 innings with the Mariners earlier this season. He will start off low on the ladder with Kansas City.