After his sophomore college season, Toppin declared for the 2020 NBA draft where he was selected with the eighth overall pick by the New York Knicks.

Toppin was born with Jewish roots in Brooklyn in New York City and originally grew up in the neighborhood of Bushwick before moving to Melbourne, Florida.

[11][12] Entering his red-shirt sophomore season, Toppin was named to the preseason first team All-Atlantic 10 and to the Karl Malone, Lute Olson and Naismith Award watch lists.

[18] In late November 2019, Toppin led Dayton to second place at the 2019 Maui Invitational Tournament, averaging 22.3 points, 7.0 rebounds, 2.3 assists and 1.3 blocks over three games.

[26] At the end of the regular season Toppin was named to the first team All-Atlantic 10 and the Atlantic 10 Conference Men's Basketball Player of the Year after averaging 20 points, 7.5 rebounds and 1.2 blocks per game with a .633 field goal percentage.

[38][39] On December 23, Toppin made his NBA debut, putting up nine points, three rebounds, and two blocks in 24 minutes, in a 107–121 loss to the Indiana Pacers.

[40][41] Toppin missed 10 games due to injury sustained in his NBA debut,[42][43] and returned on January 13, 2021, in a 109–116 loss to the Brooklyn Nets.

[46] The following season, in his sophomore year, Toppin was the winner of the 2022 NBA Slam Dunk Contest, beating Juan Toscano-Anderson in the final round.

[48] On April 10, two days later, he recorded a career-high 42 points, along with 10 rebounds and a block, shooting 16-of-28 from the field, 6-of-14 from three and 4-of-4 from the free throw line, in a 105–94 win over the Toronto Raptors.

On April 5, 2023, he tallied an impressive 32 points and 6 assists on 11-of-18 shooting from the field, 5-of-12 from three, and 5-of-5 from the free throw line in a win over the Indiana Pacers.

[50] Four days later Toppin recorded 34 points, 7 rebounds, 5 assists, and 2 steals, on 13-of-23 shooting from the field, 5-of-10 on three, and 3-of-3 on free throws in a loss to his future team, the Indiana Pacers.

On the Pacers, Toppin joined Jalen Smith, Tyrese Haliburton, Aaron Nesmith and Jordan Nwora, all selected in the 2020 NBA draft.

[54] On November 6, Toppin recorded 19 points and two steals, on 6-of-11 shooting from the field and 6-of-6 on free throws, in a 41-point blowout win over rookie Victor Wembanyama and the San Antonio Spurs.