[6][7] He was named SEC Defensive Player of the Year and earned Second-Team All-SEC honors[8] after averaging 11.9 points and a team-high 8.2 rebounds per game as a freshman in 2016–17.

On March 21, despite being credited as a potential first round lottery player, Williams decided to forgo the 2017 NBA draft and stay for another season at Texas A&M.

[9] Following Texas A&M's loss in the 2018 NCAA men's basketball tournament, Williams announced his intention to forgo his final two seasons of collegiate eligibility and declare for the 2018 NBA draft, where he was expected to be a first round selection.

[12] His reputation for missing flights and video meetings caused him to earn the moniker "Time Lord" on online Boston Celtics fandoms.

[13][14] Williams made his professional debut on October 23, 2018, playing four minutes during the Celtics' loss to Orlando Magic, before being assigned to the Maine Red Claws on November 2.

[20] On October 1, 2023, Williams, along with Malcolm Brogdon and two future first-round draft picks, was traded to the Portland Trail Blazers in exchange for Jrue Holiday.

[21] On October 25, Williams made his Trail Blazers debut, putting up 10 points and seven rebounds off the bench in a 123–111 loss to the Los Angeles Clippers.

[28] Early in his career, Williams has attracted the nickname "Time Lord" from Celtics fans, in part based upon his timekeeping faux pas upon signing with the team.