Mostert survived Saturday's final roster cuts and will serve as a backup running back and special teams contributor.
Mostert tallied 115 rushing yards, 104 receiving yards and a pair of touchdowns in two preseason games, earning him the right to begin the season on a 53-man roster for the first time in his career. The 25-year-old is slotted behind Carlos Hyde and Matt Breida on the depth chart, but he should receive a handful of touches in addition to returning kicks for the new-look 49ers.