During the 2015 summer, Diallo competed on the Under Armour Association Circuit (UAA) for the AAU team, New York Jayhawks, where he averaged 22.5 points and 5.0 rebounds per game.

In the 2016 summer, Diallo then joined the AAU team, New York Rens on the Nike Elite Youth Basketball League (EYBL) Circuit.

[9] He would start participating with the team in practices, but would not play a single game that year due to his late entry into the program.

[13] On October 16, 2018, Diallo made his debut in NBA, coming off the bench in a 100–108 loss to the Golden State Warriors with four points, a rebound, an assist and a steal.

[15] On February 16, 2019, Diallo won the NBA Slam Dunk Contest, becoming the first Oklahoma City Thunder player ever to win it.

[16] On March 13, 2021, Diallo was traded to the Detroit Pistons in exchange for Sviatoslav Mykhailiuk and a future second-round draft pick.

The next day, the Pistons announced that Diallo had suffered a grade 2 sprain in his right ankle and would be re-evaluated in three to four weeks.

[21] In a press conference the same day, however, Pistons head coach Dwane Casey stated that Diallo would miss the rest of the 2022–23 season.

[28] Diallo competed for the under-18 United States national basketball team that captured gold in the FIBA Americas Under-18 Championship game in 2016.