Mitchell (hamstring) returned to practice Sunday, Nick Wagoner of ESPN.com reports.
It's unclear how much work Mitchell may be able to handle in individual and team work, but he was spotted going through agility drills in the portion of the session open to the media. He didn't play at all this preseason as he tended to a hamstring issue that has kept him sidelined from practice since at least Aug. 10, but he now appears to be ramping up his activity with an aim to be available for a Week 1 road matchup with the Bears on Sept. 11 against the Bears. Once healthy, Mitchell will serve as the 49ers' unquestioned No. 1 running back.