He played college football for the Alabama Crimson Tide, winning a national championship in 2015 and several defensive player of the year awards in 2016.

Allen was selected by Washington in the first round of the 2017 NFL draft, with him making two Pro Bowls.

[2] Allen spent ten months in foster care before his father, US Army Sgt.

[8] As a junior in 2015, Allen started all 14 games for the Crimson Tide, who won the 2016 College Football Playoff National Championship over the Clemson Tigers by a score of 45–40.

His stock began to decline after teams became concerned when he was diagnosed with arthritis in one of his shoulders and had surgery on both, two weeks before the combine.

[10] He was ranked as the top defensive tackle by Sports Illustrated, Pro Football Focus, and NFLDraftScout.com.

[21] Allen returned for the 2018 season and started all 16 games, recording eight sacks, 61 total tackles, and 15 quarterback hits.

[25][26] He finished the 2020 season with two sacks, 63 total tackles (36 solo), one fumble recovery, and one safety.

[32] Allen punched teammate Daron Payne on the sideline during a blowout Sunday Night Football loss to the Cowboys in Week 16.

[43] Due to his own personal experience with the foster care system, Allen has often helped with homeless shelters.