He played one season of college basketball for the Kansas Jayhawks before declaring for the 2016 NBA draft, where he was selected with the 33rd overall pick by the Los Angeles Clippers.

[1] Through its international program, Diallo began attending Our Savior New American School in Centereach, New York.

[1][2] As a junior in 2014, he averaged 18.5 points, 11.2 rebounds, and 4.0 blocks per game while leading his team to a 28–3 record and a division championship.

Diallo was named MVP of the 2015 McDonald's All-American Boys Game after scoring 18 points and 10 rebounds leading the East team to a 111–91 win over the West.

[5] Diallo missed the first five games of his freshman season with Kansas after the NCAA failed to clear him to play due to eligibility concerns.

[6] On November 25, 2015, he was cleared to play for the Jayhawks in their game against Loyola on December 1, after the NCAA ultimately ruled Diallo had received a limited amount of impermissible benefits.

In April 2016, Diallo declared for the NBA draft, forgoing his final three years of college eligibility.

[13] He made his NBA debut on October 29, 2016, recording one rebound and one block in six minutes off the bench in the Pelicans' 98–79 loss to the San Antonio Spurs.

[15] On December 10, 2016, he played in just under 31 minutes off the bench and scored 19 points in the Pelicans' 133–105 loss to the Los Angeles Clippers.

[17] The following day, in the Pelicans' season finale, Diallo had 12 points and 16 rebounds in a 103–100 win over the Portland Trail Blazers.

[23] Diallo matched his season high of 18 points with perfect shooting in a 128–115 win over the Los Angeles Lakers on February 23.

[30] On December 23, 2021, Diallo signed a 10-day contract with the Detroit Pistons via the hardship exemption[30] and after his expired, he was re-acquirerd by Motor City.

[40] He had a great season individually, often leading the team in scoring, and the Osos reached the BSN finals for the first time in their history.