The Angels' trade this week for Chris Iannetta from the Rockies in exchange for promising starter Tyler Chatwood made Mathis expendable. Bobby Wilson and Hank Conger are projected as Iannetta's backups this season.
Mathis, who shared catching duties with Mike Napoli for four seasons, including three consecutive American League West championships in 2007, 2008 and 2009, was eligible for salary arbitration and would have been a potential free agent after the 2012 season.
In Mathis' 375 career starts behind the plate, the Angels won 216 games for a .576 winning percentage. The club's overall winning percentage in that period was .564.
Mills, 26, spent time at Triple-A Las Vegas and Toronto in 2011. He started a team-high 24 games for Las Vegas, going 11-9 with a 4.00 ERA in 157 1/3 innings. He had 136 strikeouts against only 39 walks.
With the Jays, Mills made five appearances, four as a starter. He was 1-2 with a 9.82 ERA in 18 1/3 innings. 14 career appearances, including nine starts, for the Jays with an 8.57 ERA.
A native of Mesa, Ariz., Mills was taken by Toronto in the fourth round of the 2007 First-Year Player Draft out of the University of Arizona.
Mathis, 28, shunned a football scholarship to Florida State University and signed with the Angels as a first-round pick in 2001 out of Marianna, Fla., High School.
Known as a premier defensive player, Mathis struggled offensively. In 93 games in 2011, he batted .174 with 12 doubles, three home runs and 22 RBIs. He made a team-high 79 starts behind the plate and appeared in 91 games at the position. Mathis ranked tied for first among American League catchers with a career-best .995 fielding percentage in 568 total chances.
Mathis is a career .194 hitter in 426 games. His shining moments offensively came during the 2009 postseason when he had eight hits in 15 at-bats, including five doubles, and had a walk-off run-scoring double in the AL Championship Series against the Yankees. He is 9-for-20 (.450) in 20 postseason at-bats.
In the Fielding Bible section of the 2012 Bill James Handbook, Mathis is rated the fourth best defensive catcher in the Majors by a panel of experts, behind Matt Wieters, Yadier Molina and Carlos Ruiz.