Now, Mostert and the 49ers are headed to Super Bowl LIV.
Mostert led the team with 772 rushing yards and eight touchdowns this season while averaging 5.6 yards per carry. It wasn't until Sunday, however, that he became a household name. In the biggest game of his life, and the biggest in the history of Levi's Stadium, Mostert carried the 49ers offense to a 37-20 win. The running back had 29 carries for 220 rushing yards, setting a franchise record for the most rushing yards in a single game and the second-most in any NFL playoff game, second only to Eric Dickerson's 248 in 1986.
Our friends over at Niners Wire tweeted out a list of all the running backs in NFL history to rush for 200-or-more yards in a single game and score four rushing touchdowns. Mostert was the lone name on the list.
That tells you how historic a performance Mostert had in the NFC Championship Game against the Green Bay Packers.
The 49ers didn't need to pass the football much. Quarterback Jimmy Garoppolo attempted just eight passes, completing six for 77 yards. It wasn't because head coach Kyle Shanahan lacks faith in his starting quarterback, who in Week 14 went toe-to-toe with Drew Brees of the New Orleans Saints and won. It was because the 49ers offense didn't need to pass. Green Bay wasn't able to stop Mostert, so why switch it up?
Mostert recalls the moment he realized exactly what was going on in the game.
"I'm sitting down, just got done with a drive," Mostert said Tuesday on the ESPN's Golic and Wingo show. "It's like the end of the third quarter, going into the fourth quarter, and I look up at the scoreboard, and they display the stats per team, like rushing and passing. I look at it, I'm like, 'Man, 200 yards rushing. We're really running the ball. And only 40 yards passing.'
"Jimmy comes up to me, and he's like, 'Hey, Heem, how many yards you got?' I'm like, 'Man, dude, I don't even know.' I know we're running the ball a lot, and I'm like, 'Hey, look, I think I've got like 200 rushing yards.' He's like, 'Oh, no way. Really?' I'm like, 'Yeah.'
"And he's like, 'How many passing yards we have?' Mind you, at this time, he hasn't looked up at the scoreboard to see the stats. I'm like, 'Jimmy, look up.' I pointed at the scoreboard. He's like, 'Dude, I've only thrown the ball for 55 yards?' He could not believe it."
Garoppolo went on to note that at least his completion percentage was high.
Mostert remembers Garoppolo in the huddle warming up his arm. After all, at one point, he went over an hour-and-a-half without throwing a pass. The quarterback probably got more work in during pre-game warmups.
"Literally, he was in the huddle just warming up his arm, doing arm circles and stuff like that," Mostert continued. "I'm like, 'Jimmy, you know it's going to be a run.'"