Grantham suffered a torn ACL in a game against Notre Dame on January 21, 2018, prematurely ending his college career.

[2] After going undrafted in the 2018 NBA draft, Grantham signed with the Oklahoma City Thunder on October 10, 2018.

[6] Grantham made his NBA debut with the Thunder on February 7, 2019, playing in only two minutes of action under a blowout 117–95 win over the Memphis Grizzlies.

[10] On December 15, Grantham posted 25 points, nine rebounds, two assists and a steal in a loss to the Santa Cruz Warriors.

[13] On 29 July 2023, Grantham signed with London Lions of the British Basketball League (BBL).