Diallo attended John Bowne High School in Flushing, New York during his freshman and sophomore year. As a sophomore, he averaged 17.1 points, 6.6 rebounds per game, and 2.7 assists. During the 2015 summer, Diallo competed on the Under Armour Association Circuit for the AAU team, NY Jayhawks. He averaged 22.5 points and 5.0 rebounds per game on 2015 Under Armour Association Circuit. Later that summer Diallo was invited to both NBPA Top 100 and Adidas Nations camps. Diallo transferred to Putnam Science Academy in Putnam, Connecticut prior to his Junior Year. As a junior, he averaged 17.0 Points per game, and 4.0 rebounds while leading Putnam to a overall record. In the 2016 summer, Diallo then joined the AAU team, New York Rens on the Nike Elite Youth Basketball League Circuit. He averaged 18.6 points, and 2.1 assists while leading the Rens to the season-ending Peach Jam. As a senior in 2016-17, he averaged 19.0 points, 6.0 rebounds, and 3.0 assists where he led the Mustangs to a record. Diallo was considered one of the top players in the 2017 recruiting class by Scout.com, Rivals.com and ESPN. Diallo was heavily recruited by six schools, the University of Kentucky, The University of Arizona, Indiana University, The University of Kansas, Syracuse University, and the University of Connecticut.
College career
On January 7, 2017, Diallo committed to the University of Kentucky. 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. Diallo was one of a record-high 182 players to declare for the 2017 NBA Draft, despite not having played a single college game in the process. On May 24, 2017, Diallo announced that he would return to Kentucky to play in their 2017-2018 season, despite draft scouts saying he could have been taken in the first round of the draft that year. Diallo had season averages of 10.0 points per game, and 3.6 rebounds per game.
On June 21, 2018, Diallo was selected with the 45th pick in the 2018 NBA draft by the Brooklyn Nets. His draft rights were subsequently traded to the Charlotte Hornets, and then to the Oklahoma City Thunder. On July 28, 2018, the Oklahoma City Thunder announced that they had signed with Diallo. 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. On November 19, 2018, Diallo scored his career high 18 points with two steals, a rebound, and an assist in a 113–117 loss to the Sacramento Kings. On February 16, 2019, Diallo won the NBA Slam Dunk Contest, becoming the first Oklahoma City Thunder player ever to win it. For one dunk he jumped over Shaquille O'Neal, and did the "honey dip" dunk popularized by Vince Carter, and displayed a Superman undershirt while hanging from the rim. In another dunk, he jumped over Quavo who held the ball above his head and finished it with two hands.
