49ers sign DT Earl Micthell for postseason
Mitchell spent the previous two seasons with the 49ers but has not played since being cut by Seattle just before the start of this season. Mitchell had announced his retirement in November but has decided to help the Niners in their playoff run.
San Francisco is the top seed in the NFC and has a bye this week. The Niners will host either Minnesota, Seattle or Philadelphia on Jan. 11 in the division round.
Mitchell adds depth in the middle of the defensive line after D.J. Jones was lost to a season-ending ankle injury.
Mitchell has played 130 games over nine seasons with Houston, Miami and San Francisco. He has 324 tackles and 6 1/2 sacks but is mostly used as a run stuffer.
Exum played seven games this season, mostly appearing on special teams.