The Cubs selected Thornton's contract from Triple-A Iowa on Monday.
The move corresponds with Charlie Barnes being designated for assignment and the Cubs placing Matthew Boyd (knee) on the 15-day injured list. Thornton was activated from Iowa's 7-day injured list in late April after fully recovering from surgery in August to repair a torn left Achilles tendon. The 32-year-old right-hander has appeared in four Triple-A games, allowing two runs on five hits and three walks while striking out five across 5.2 innings.