Allen is a New Jersey native but played three years of high school football in Alabama before moving back to Jersey for his senior year (43.5 tackles for loss, 22.5 sacks). Kentucky convinced him to come back to the South for college. He played in 12 games as a reserve in 2015 (four tackles, 1.5 for loss) before turning up his game as a sophomore. started nine of 13 games in 2016, accumulated 62 tackles, 8.5 for loss, and a team-high seven sacks. He also led the SEC with four forced fumbles. Allen started all 13 games as a junior, topping the Wildcats with 9.5 tackles for loss and seven sacks while being credited with 65 tackles, three pass breakups, and two forced fumbles. High expectations were set for Allen in 2018 and he exceeded them, winning the Bednarik Award and the Nagurski Trophy for being one of the top defenders in college football. The First-team Associated Press All-American and first-team All-SEC pick led the Wildcats with 88 tackles in 13 games, tied for sixth nationally with 21.5 tackles for loss and ranked second in the FBS in both sacks (17) and forced fumbles (five).