He was selected by the Miami Dolphins in the second round of the 2017 NFL draft, and later played for the Las Vegas Raiders and the New England Patriots.

He also participated at Ohio State's highly anticipated pro day, but opted to only run positional drills.

[6] Seven general managers and nine head coaches attended Ohio State's pro day to scout McMillan and ten other prospects.

[7] McMillan had private workouts and visits with 11 teams that included the Cincinnati Bengals, New England Patriots, Miami Dolphins, Pittsburgh Steelers, Kansas City Chiefs, Washington Redskins, Baltimore Ravens, Detroit Lions, New York Jets, Oakland Raiders, and New Orleans Saints.

[19] McMillan entered training camp slated as the starting strongside linebacker after Koa Misi retired due to a neck injury.

McMillan remained the apparent front-runner throughout camp, but saw minor competition from Mike Hull and Neville Hewitt.

[24] He made his professional regular season debut and first career start in the Dolphins’ season-opener against the Tennessee Titans and recorded six combined tackles during their 27–20 victory.

[25] On December 23, 2018, McMillan collected a season-high ten combined tackles (six solo) during the Dolphins’ 17–7 loss against the Jacksonville Jaguars in Week 16.

[36] On May 25, it was announced that McMillan had suffered a partially torn Achilles tendon at organized team activities and would be placed on season-ending injured reserve, missing the entirety of his 2023 season once again.

[40] On December 24, 2024, McMillan was signed to the Tennessee Titans practice squad, and promoted to the active roster four days later.