McMillan played college football at Ohio State. As a freshman in 2014, he appeared in 13 games making 54 tackles, 2.5 sacks, an interception returned for 24 yards, and a pass defended. As a sophomore in 2015, he played 13 games with 119 tackles, 1.5 sacks, four passes defended, and a fumble recovery. As a junior in 2016, he played 13 games with 102 tackles, two sacks, five passes defended, and two forced fumbles. After his junior year, McMillan declared his intentions to enter the 2017 NFL Draft.
Professional career
McMillan received an invitation to the NFL Combine and completed all of the combine and positional drills. He also participated at Ohio State's highly anticipated pro day, but opted to only run positional drills. Seven general managers and nine head coaches attended Ohio State's pro day to scout McMillan and ten other prospects. 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. The majority of NFL draft experts and analysts projected McMillan to be selected in the second or third round. He was ranked the second best inside linebacker in the draft by NFL analyst Bucky Brooks, the third best inside linebacker by NFLDraftScout.com, the fifth best linebacker by Sports Illustrated, and was ranked the tenth best linebacker in the draft by ESPN.
2017
The Miami Dolphins selected McMillan in the second round of the 2017 NFL Draft. He was the seventh linebacker selected in 2017. On May 5, 2017, the Miami Dolphins signed McMillan to a four-year, $4.69 million contract with $3.16 million guaranteed and a signing bonus of $1.55 million. McMillan entered training camp slated as the starting strongside linebacker after Koa Misi retired due to a neck injury. He impressed coaching staff and learned all three linebacker positions. McMillan remained the apparent front-runner throughout camp, but saw minor competition from Mike Hull and Neville Hewitt. On August 10, 2017, McMillan made his professional debut in the Miami Dolphins’ first preseason game against the Atlanta Falcons, but left the game after suffering an apparent knee injury. The following day, it was confirmed that he had a torn an ACL and had abruptly ended his rookie season. On September 2, 2017, the Miami Dolphins officially placed McMillan on injured reserve.
McMillan entered training camp slated as the de facto starting middle linebacker. Head coachAdam Gase named McMillan the starting middle linebacker to begin the regular season, along with outside linebackers Kiko Alonso and rookie Jerome Baker. He made his professional regular season debut and first career start in the Miami Dolphins’ season-opener against the Tennessee Titans and recorded six combined tackles during their 27–20 victory. On December 23, 2018, McMillan collected a season-high ten combined tackles during the Dolphins’ 17–7 loss against the Jacksonville Jaguars in Week 16. He started in all 16 games in 2018 and recorded 105 combined tackles, two forced fumbles, and one pass deflection. He earned an overall grade of 63.0 from Pro Football Focus, which ranked 55th among all qualified linebackers in 2018.
2019
McMillan entered the 2019 season as the Dolphins starting middle linebacker. He played in 13 games before suffering a hamstring injury in Week 15. He was placed on injured reserve on December 17, 2019.
