Milligan, 5-10, 200 pounds, spent time on the Colts' active roster and practice squad in 2019 before being placed on the team's Injured Reserve list on December 18. He saw action in 11 games (one start) and totaled 10 tackles (eight solo). In 2018, Milligan spent time on Indianapolis' active roster and practice squad but did not see game action. He also spent time on the Detroit Lions practice squad in 2018. In 2017, Milligan spent time on the Lions' active roster and practice squad but did not see game action. He participated in the Dallas Cowboys' 2016 offseason program and training camp before being waived on August 16, 2016. Milligan was originally signed by the Cowboys as an undrafted free agent on May 9, 2016.