Williams was elected as the First Vice President of the National Basketball Players Association in February 2023, replacing Andre Iguodala in the role.

[2][3] He attended Providence Day School in Charlotte, where he was teammates with future and fellow NBA player Devon Dotson.

[12] In an effort to garner support for teammates making the 2020 NBA All-Star Game, Williams pledged to dye his hair pink if Jaylen Brown, Jayson Tatum and Kemba Walker all received a selection.

On May 15, Williams scored a career-high 27 points in a decisive game 7 win over the Milwaukee Bucks in the Eastern Conference Semifinals.

[18] On May 19, 2023, during game 2 of the Eastern Conference Finals versus the Miami Heat, Williams had nine points, two rebounds, two assists and one block in 26 minutes off the bench.

[19] After Williams scored on a three-pointer to give Boston a 96–87 lead with 6:37 left, he mouthed some words to Heat forward Jimmy Butler while in transition to defense.

Williams was driving to the basket late in the fourth quarter, losing his footing after bumping into Bucks center Brook Lopez, where he landed awkwardly with a long step and collapsed onto the court.

[28] His father, Gilbert, is a jazz artist and former college basketball player, who has worked as a bodyguard for musicians, including Prince.

[30] Turning down offers from Ivy League schools Harvard and Yale,[31] Williams graduated from Tennessee in three years with a degree in business.