Leonard played two seasons of college basketball for the San Diego State Aztecs and was named a consensus second-team All-American as a sophomore.

[12] During his sophomore season, Leonard averaged 15.7 points and 10.4 rebounds per contest[9] as the Aztecs finished with a 34–3 record and won back-to-back conference tournament championships.

On February 1, 2020, San Diego State retired Leonard's number 15,[16] making him the first Aztecs men's basketball player to have his jersey raised to the rafters.

[20][19] Leonard was selected with the 15th overall pick in the 2011 NBA draft by the Indiana Pacers but was traded that night to the San Antonio Spurs along with the rights to Erazem Lorbek and Dāvis Bertāns in exchange for George Hill.

[41] He returned to action on January 16, 2015, after missing 15 games, recording 20 points, 4 rebounds, 5 assists and 3 steals to lead the Spurs to a 110–96 win over the Portland Trail Blazers.

[59] On January 14, 2017, he set a new career high with 38 points in a 108–105 loss to the Phoenix Suns, becoming the first Spur to record three consecutive 30-point games since Tony Parker in 2012.

[61] Two days later, he set a new career high with 41 points in a 118–115 overtime win over the Cleveland Cavaliers, becoming the first San Antonio player to score at least 30 in six straight games since Mike Mitchell in 1986.

[65] On March 6, after being named Player of the Week for the fourth time in his career, Leonard scored 39 points to lead the Spurs to a 112–110 win over the Houston Rockets.

[71] The Spurs went on to advance to the Western Conference finals with a 4–2 triumph over the Houston Rockets in the second round, despite playing without Leonard in game 6 due to an ankle injury.

[72] In the third quarter of game 1 of the Western Conference finals against the Golden State Warriors, Leonard landed on Zaza Pachulia's foot after attempting a field goal and re-aggravated his existing ankle injury.

[83] A month later, on July 18, Leonard and teammate Danny Green were traded to the Toronto Raptors in exchange for DeMar DeRozan, Jakob Pöltl and a protected 2019 first-round draft pick.

[84] It was a risky move for the Raptors and their president, Masai Ujiri, given the concerns over Leonard's health and the possibility of him leaving as a free agent at the end of the season.

[92] In the deciding game 7, Leonard hit a shot from the corner at the buzzer that bounced off the rim four times before falling to give the Raptors a 92–90 victory over the 76ers to advance to the Eastern Conference finals.

In game 3 of the Eastern Conference finals, Leonard scored 36 points, including eight in the second overtime, to help the Raptors beat Milwaukee 118–112 to cut the Bucks' series lead to 2–1.

[97] Leonard put an exclamation point on the conference victory with a huge dunk on Giannis Antetokounmpo that was set up by a Kyle Lowry steal.

[102] On July 10, 2019, Leonard signed with his hometown team, the Los Angeles Clippers, to a reported three-year, $103 million contract, which included an opt-out clause in 2021.

[108] On December 11, Leonard returned to Toronto for the first since joining the Clippers and was met with a tribute and a standing ovation from the crowd before scoring 23 points and recording six assists in 112–92 win over the Raptors.

[122] After being absent for two games due to the injury, Leonard returned to the Clippers wearing a clear plastic mask with a 128–105 win over the Portland Trail Blazers on December 30, recording a game-high 28 points and seven assists.

[125] Leonard was ruled out for the remainder of the series with a knee sprain, as the Clippers won the next two games and advanced to the first Western Conference finals in franchise history.

[126] The Clippers' playoffs run was then cut short by the Phoenix Suns in six games, with Leonard unable to recover and missing the entire series.

[132] Leonard made his return from injury on October 3, 2022, putting up 11 points, four rebounds, two assists, and two steals in a 102–97 preseason win over the Trail Blazers.

[133] On October 20, Leonard made his regular season return, logging 14 points, seven rebounds, and two assists off the bench in a 103–97 win over the Los Angeles Lakers.

[137] On January 20, Leonard scored a then season-high 36 points on 13-of-18 shooting, 4-of-5 from three and 6-of-6 from the free throw line and delivered seven assists in an 131–126 win over his former team, the San Antonio Spurs.

[138] On February 14, Leonard scored 33 points, tying his career high with seven three-pointers in a 134–124 win over the reigning champions Golden State Warriors.

[139] On February 24, Leonard scored a season-high 44 points on 16-of-22 shooting, 6-of-9 from three, 6-of-6 from the free throw line in a 176–175 double overtime loss against the Sacramento Kings.

[142] In game 1 of the Clippers' first-round playoff series against the Phoenix Suns, Leonard posted a game-high 38 points, five rebounds and five assists in a 115–110 win.

According to Clippers head coach Tyronn Lue, Leonard suffered a "setback" in July while participating in United States national team camp in Las Vegas prior to his withdrawal from the 2024 Summer Olympics.

[162] Known early in his career primarily as an elite defender and floor spacer, Leonard began taking on a larger offensive responsibility from the 2015–16 season on and quickly established himself as one of the most efficient scorers in the league.

[159][163][164][165] He has developed a versatile offensive game, capable of hitting shots reliably from midrange and 3-point range, posting up his opponents and shooting pull-up jumpers and fadeaways.

[190] On May 3, 2021, Leonard took to Instagram Live for the first time ever, chatting with high school basketball player Mikey Williams and announced that he will release a hip hop album titled Culture Jam sometime in 2021.