During the 1975-76 NBA season, Carr played for the Spirits of St. Louis in the ABA, averaging 12.2 points and 6.2 rebounds per game, and was named to the ABA's All-Rookie Team. The Spirits of St. Louis were one of two ABA teams (the Colonels being the other) that did not join the NBA in the ABA-NBA merger, and as a result Carr joined the NBA as a member of the Detroit Pistons from 1976–79 and the Celtics from 1979–85. He averaged 9.7 points and 4.3 rebounds per game during his NBA career, while being selected to the All-Defense second team during the 1979 season after leading the league in steals.
He was best known for the steal and dunk he made in Game 4 of the 1984 NBA Finals, which sealed the victory for Boston, and eventually won a title for them. He is also famous for waving a towel during crucial situations to fire up the Celtics.
Carr later became the General Manager of the Celtics in 1994. He later took over as coach for the 1995-96 and 1996-97 seasons. In his last year as coach, the Celtics had the worst record in team history, winning just 15 games and losing 67, which some thought was an effort to get a stronger draft position and poise the team for a comeback under famed college coach Rick Pitino. He was replaced at the end of season by Pitino, who was unable to restore the team to the glory of Carr's playing days. After the 1997 season, he became the Celtics' Director of Corporate Development.
Carr later became president of the WNBA's Charlotte Sting as part of a failed attempt to become the owner of an expansion NBA team in Charlotte, along with Steve Belkin and former teammate Larry Bird. He was given a small investment stake in the Charlotte Bobcats when Bob Johnson was selected to have the NBA franchise in Charlotte. Subsequently, Bob Johnson sold the team and ML Carr no longer has a relationship with the Bobcat franchise.