Mullens told reporters Tuesday that 49ers head coach Kyle Shanahan informed him that he will start Monday's game.
Mullens took first-team reps in practice Tuesday, followed by C.J. Beathard and Tom Savage.
Mullens earned the starting job based on his performance in Thursday's victory over the Oakland Raiders, and his ascent to the starting quarterback spot is quite a story.
Mullens was signed by the 49ers in 2017 as an undrafted free agent out of Southern Mississippi. He spent the entire 2017 season on the practice squad and was still on the 49ers' practice squad this season before being promoted to the active roster when Jimmy Garoppolo suffered a season-ending knee injury in the third game.
Mullens was the backup to Beathard until he suffered a thumb injury that enabled Mullens to make his NFL debut as the starter against Oakland.
Mullens threw for 262 yards with three touchdown passes and no interceptions to guide the 49ers to a 34-3 rout of the Raiders.
Shanahan said after that game that he had not decided whether Mullens or Beathard would start against the Giants.
"Yeah, definitely we'll consider it," Shanahan said after the game regarding Mullens' chances of remaining the starter. "Definitely not thinking about that right now, but our whole team played very well. Nick definitely did."
Beathard has won one of his 10 career starts and is 0-5 this season. Mullens is 1-0.