[7][8] As a true freshman in 2014, Allen competed with Kenny Hill before the season for the starting quarterback position.

[16] Following the transfer of Kenny Hill to Texas Christian University, Allen was positioned to be the starter for the Aggies in 2015.

[17] On August 24, 2015, Allen was named the starting quarterback against Arizona State with competition from incoming freshman Kyler Murray.

[19] The Friday before the following game, head coach Kevin Sumlin named Murray the starter[20] and Allen started to practice with the third team.

Against Alabama, Allen said that he suffered an AC sprain in his throwing shoulder but told the coaching staff he was well enough to face Ole Miss.

Allen would then miss playing in the 35–28 win on October 31 against South Carolina and the Aggies loss to Auburn (26–10).

[21] This decision made Allen unavailable to play in the Aggies' Music City bowl game.

On January 11, 2018, Allen announced his decision to forgo his final year of eligibility and declared for the 2018 NFL draft.

[32] On December 20, he was promoted to the active roster to back up Taylor Heinicke after starter Cam Newton was shut down for the season.

[36] Allen made his first start of the 2019 season in Week 3 against the Arizona Cardinals due to a foot injury to Cam Newton.

[38] He helped lead the Panthers to victories over the Jacksonville Jaguars and Tampa Bay Buccaneers over the following two weeks.

[44] On December 16, 2019, one day after the game, the Panthers announced that Allen would be benched for rookie quarterback Will Grier in Week 16 against the Indianapolis Colts.

[47] On March 24, 2020, Allen was traded to the Washington Football Team for a fifth-round pick in the 2020 NFL draft.

[48] Allen was named the starter after Dwayne Haskins was benched prior to their Week 5 game against the Los Angeles Rams.

[49] Against the Rams, he ran for a touchdown before missing the rest of the game after suffering a helmet-to-helmet hit by Jalen Ramsey in the second quarter.

[53][54] Allen played for the first time in the 2021 season in the Week 14 loss against the Dallas Cowboys, in relief of Taylor Heinicke who left the game in the fourth quarter due to injury.