Smart was drafted with the sixth overall pick in the 2014 NBA draft by the Boston Celtics. He initially came off the bench for the Celtics before starting games regularly in 2018 and becoming the starting point guard in 2021.
Smart was named the NBA Defensive Player of the Year for the 2021–22 season, becoming only the sixth guard in NBA history to win the award.
He has also been named to the NBA All-Defensive First Team thrice and is a three-time NBA Hustle Award winner, winning it in 2019, 2022, and 2023. Smart helped the Celtics reach the NBA Finals in 2022 before being traded to the Grizzlies in 2023.
