[3] Smart attended Edward S. Marcus High School in Flower Mound, Texas, along with one of his future Oklahoma State teammates, Phillip Forte.

[5] Smart went on to set a new Team USA U18 record for steals over a five-game period when he grabbed 18 during the 2012 FIBA Americas U18 Championships in Brazil.

Smart and the Cowboys earned a trip to the NCAA Tournament that year, clinching a #5 seed in the Midwest Region.

[10] On November 19, 2013, Smart tied an OSU single game scoring record with 39 points leading the #7-seed Oklahoma State Cowboys past the #11-seed Memphis.

At a press conference the following afternoon, Smart would not comment on that element of the altercation, and coach Travis Ford chose not to address it.

[25] Three days later, Smart was suspended for one game without pay for hitting San Antonio Spurs forward Matt Bonner in the groin the previous night.

[27] On July 16, 2015, while playing for the Celtics at the 2015 Las Vegas Summer League, Smart dislocated two fingers on his right hand.

[33] On December 25, he scored 15 points and made a tie-breaking three-pointer with 47 seconds left to help the Celtics claim a 119–114 victory over the New York Knicks.

[35] In Game 3 of the Eastern Conference Finals on May 21, Smart, who started in place of the injured Isaiah Thomas, made seven three-pointers and scored 27 points to help the Celtics defeat the Cleveland Cavaliers 111–108.

On November 27, 2017, Smart scored a season-high 23 points, making 6-of-9 from three-point range, in a 118–108 loss to the Detroit Pistons.

[37] He missed 11 games between January 24 and February 14 after cutting his hand on glass at the team hotel in Los Angeles.

During the summer of 2021, new head coach Ime Udoka named Smart the team's starting point guard;[55][56] he remained in that role throughout the season and during the Celtics' 2022 playoff run.

[55] Both Fox Sports and Deadspin have credited Smart's move to the starting point guard role as a major catalyst for the team's successful 2021–22 season.

[59] Smart was named the 2021–22 NBA Defensive Player of the Year on April 18, 2022, the first guard to win the award since Gary Payton won it in 1996.

[60] Smart became the fifth guard in NBA history (alongside Michael Jordan, Gary Payton, Sidney Moncrief, and Alvin Robertson) to win the award.

[62] On May 19, 2022, in Game 2 of the Eastern Conference Finals, Smart recorded a playoff career-high 12 assists along with 24 points, nine rebounds, and three steals in a 127–102 victory over the Miami Heat to tie the series at 1–1.

[64] In Game 3 of the Finals, Smart posted 24 points, seven rebounds, and five assists in a 116–100 victory over the Golden State Warriors.

[67] On June 22, 2023, Smart was traded to the Memphis Grizzlies as part of a three-team deal that sent Kristaps Porziņģis to the Celtics and Tyus Jones to Washington.

[68][69] On October 25, 2023, Smart made his Grizzlies debut, putting up 17 points, three assists, and two steals in a 111–104 loss to the New Orleans Pelicans.

[70] At the 2024 NBA All-Star break, Smart averaged a career best 14.4 points alongside 2.0 steals while playing 30.2 minutes per game.

Smart, at a height of 6'3" with a 6'9" wingspan, is capable of guarding all five positions, enabling him to play a switching defense with historic efficiency.

[75] Due to his physicality, quick hands, and elite basketball IQ,[76] many consider Smart to be one of the most versatile and consistent defenders in the NBA.

He often guards opposing players taller than he is, using his physicality to make them uncomfortable and often causing turnovers or missed shots.