Selected by the Raiders in the second round (40th overall) of the 2019 NFL Draft, April 26, 2019. The first cousin of former Heisman Trophy-winner at Louisville and current Baltimore Ravens quarterback Lamar Jackson decided to join his relative in the NFL instead of returning to Clemson for his senior season. Mullen was named second-team All-American by USA Today and was a top 100 overall prospect nationally out of Fort Lauderdale, starring as a receiver his senior year. He played as a reserve defensive back and on special teams coverage units as a freshman in 2016 (15 tackles, one pass breakup). Mullen started 12 of 13 games played the following season, receiving honorable mention All-ACC accolades for his play (42 tackles, three interceptions, seven pass breakups). He was a second-team all-conference pick in 2018, starting 14 of 15 games played for the national champions. Mullen posted 37 stops, four for loss, an interception and four pass breakups as a junior, finishing the year as the title game Defensive MVP with 6 tackles, a sack, an interception and a forced fumble to help shut down Alabama's offense in the second half.