After playing one season of college basketball for the UCLA Bruins, he was selected by the Indiana Pacers in the first round of the 2017 NBA draft with the 18th overall pick.

[7] As a junior, Leaf averaged 27.4 points, 14.2 rebounds, 4.8 assists, 1.8 steals and 2.7 blocks per game, leading Foothills Christian to the San Diego Section Division II championship.

[8][18] A consensus five-star and overall top-20 recruit,[19][20] he joined his Amateur Athletic Union (AAU) teammates from the Compton Magic, Ike Anigbogu and Kobe Paras, who also committed to UCLA.

[24] On February 1, 2017, Leaf scored a career-high 32 points on 14-for-18 shooting and added 14 rebounds in a 95–79 win over Washington State, which ended a two-game losing streak for No.

[25] He missed the regular season finale with a sprained left ankle, which he suffered five minutes into the prior game against Washington.

[27][28] He received honorable mention from the Associated Press (AP) for their All-American team,[28] and was named first-team All-Pac-12 along with teammates Lonzo Ball and Bryce Alford.

[30] Leaf was selected by the Indiana Pacers in the first round of the 2017 NBA draft with the 18th overall pick, and he signed a three-year contract worth $7,249,200.

In December 2017, Leaf was assigned to the Fort Wayne Mad Ants of the NBA G League and averaged 23.3 points and 8.3 rebounds in a three-game stint before returning to the Pacers.

[34] In 2018–19, Leaf's minutes were limited playing behind big men Myles Turner, Domantas Sabonis, Thaddeus Young and Kyle O'Quinn.

Leaf was forced into the game early after Turner and O'Quinn got into foul trouble guarding the Wolves' Karl-Anthony Towns, and held his own defensively against the All-Star center.

[39] On November 3, 2019, Leaf had 13 points and a career-high 15 rebounds while getting extra time as a backup center in a 108–95 win over the Chicago Bulls.

[45] On November 25, 2020, Leaf was traded along with a 2027 second-round draft pick to the Oklahoma City Thunder in exchange for guard Jalen Lecque.

[49] On January 12, 2022, Leaf joined the Guangzhou Loong Lions of the CBA,[50] and averaged 25 points, 11.7 rebounds and 1.7 blocks per game.