As a sophomore, he received significantly more playing time, as he registered 42 tackles, 3 interceptions and seven pass breakups in 13 games with 12 starts.

As a junior, he collected 37 tackles (4 for loss), one interception, 4 pass defensed and 2 sacks in 15 games with 14 start.

Following the season, Mullen announced that he would forgo his senior year and declared for the 2019 NFL draft.

[2] He finished his college career with 94 tackles (4.5 for loss), 4 interceptions, 12 pass breakups and one forced fumble in 41 games with 26 starts.

[7] In week 16 against the Los Angeles Chargers, Mullen suffered a head injury, which caused him to be carted off the field.

In Week 11 against the Kansas City Chiefs, Mullen recorded his first interception of the season off a pass thrown by Patrick Mahomes during the 35–31 loss.

On October 9, 2021, he suffered a foot injury in Week 4 and was placed on injured reserve, ending a 36 consecutive games streak.

On December 13, 2022, he was waived a day after a 13-27 loss against the New England Patriots, where he committed a costly facemask penalty that contributed on a touchdown drive.

[13] He was acquired to provide depth at cornerback after injuries to Anthony Brown and Jourdan Lewis.