The Reds selected Mills' contract from Triple-A Louisville on Tuesday.
Mills signed a minor-league deal with Cincinnati in May and will now be called upon to provide depth to the Reds' heavily injured pitching staff. The 31-year-old righty has made two appearances in Triple-A, allowing three earned runs on five hits and three walks while striking out just one batter through six innings. Though Mills will likely work out of the bullpen initially, the Reds recently optioned Levi Stoudt to Triple-A and could have an opening in the rotation the next time the club requires a fifth starter July 4 in Washington.