The Blue Jays claimed Mills off waivers from Oakland on Thursday, bringing him back to the team that drafted him in 2007.
Manager John Gibbons says Mills will serve as the long man out of the bullpen, allowing right-hander Todd Redmond to pitch later in games.
"He's been pitching pretty damn good," Gibbons said of Redmond. "We'll give him some tougher situations, see how he does."
Mills went 1-1 with a 4.41 ERA in three starts with the Athletics. He has a career record of 4-4 with a 6.98 ERA in 18 games with Toronto, the Los Angeles Angels and Oakland.
"He's a left-hander, he's a strike-thrower," said Gibbons.
"We need to adds some depth, some arms," he added.
Jenkins had no record and a 3.72 ERA in 15 appearances with the Blue Jays this season before he was sent down Friday.
Gibbons also provided injury updates Friday, saying first baseman Edwin Encarnacion (quad strain) was close to getting on an exercise bike "and start pushing it a little bit."
Third baseman Brett Lawrie (fractured finger) is making progress but "still that's up in the air." Same for first baseman-DH Adam Lind (fractured foot).
Pitcher Brandon Morrow (torn tendon in finger) is slated to go to Florida next week to continue his rehabilitation.