[2] After completing his senior season at Arizona, Mills would once again be selected by the Blue Jays, this time in the fourth round (145th overall) of the 2007 amateur entry draft.

Splitting his season with three teams (the Single–A Lansing Lugnuts, the High–A Dunedin Blue Jays, and the Double–A New Hampshire Fisher Cats), Mills was 13–5 in 27 games (all starts), with 159 strikeouts over 147+1⁄3 innings pitched, with an ERA of 1.95.

Advancing to the Triple–A Las Vegas 51s for 2009, Mills at the time of his promotion was leading the team in innings pitched with 76+1⁄3, and had a 1–8 record, a 4.48 ERA, and 65 strikeouts.

[3] The Blue Jays had suffered a string of injuries to many of their pitchers from early 2009 onwards, leading to several promotions from within their minor-league system.

The next day, he made his major league debut by taking over Casey Janssen's scheduled start on the road against the 2008 World Series champion Philadelphia Phillies, giving up four earned runs in 3+2⁄3 innings.

[5][6] He made his only appearance for the Angels on July 8 and earned his 3rd career win, pitching five scoreless innings, with no walks and six strikeouts, and three hits allowed.

[8][9] Mills was assigned to the Rangers' Triple-A team, the Round Rock Express of the Pacific Coast League (PCL), after clearing waivers.

He had an opt-out clause in his contract with the Brewers that allowed Mills to leave the organization for zero compensation if another team put him on their active roster.