The 29-year-old Mills started three games for Oakland this season, going 1-1 with a 4.41 ERA.
He opened the season with the Milwaukee Brewers' Triple-A affiliate in Nashville. He had a 4-2 record and 1.56 ERA before being traded to Oakland.
Originally drafted in the fourth round in 2007 by the Blue Jays, Mills has a career record of 4-4 with a 6.98 ERA with Toronto, the Los Angeles Angels and Oakland.
To make room on the 40-man roster, the Blue Jays designated right-hander Deck McGuire for assignment. The team will make a corresponding move to the 25-man roster once Mills reports.