Mostert registered a team-high 772 rushing yards and eight touchdowns last season despite not recording one start. He broke out for 220 yards and four scores in the NFC Championship Game against the Green Bay Packers.
The 28-year-old was entering the second season of a three-year, $8.7-million extension signed in March 2019, one that his agent said paid Mostert for special teams.
Mostert played in 34% of the offensive snaps and 45% of special-teams snaps in 2019.
His rapid rise came somewhat unexpectedly. Mostert started the campaign playing behind Matt Breida and Tevin Coleman but was the lead dog by the end of the year. He scored touchdowns in each of the last six games of the regular season.
He took a back seat again in the divisional round as Coleman drew 22 carries for 105 yards and two touchdowns against the Minnesota Vikings, but Mostert ran through the Packers the following week in a 37-20 victory that sent the 49ers to the Super Bowl.
San Francisco traded Breida to the Miami Dolphins in April for a fifth-round pick.