Stewart earned McDonald's All-American honors and was named Mr. Basketball USA and Naismith Prep Player of the Year.

[5] In October 2016, he broke his tailbone during United States national under-16 team tryouts and consequently missed most of his sophomore season.

[9] In his senior season with La Lumiere, he averaged 18.1 points, 11.3 rebounds, and 2.9 blocks per game, helping his team to a 30–1 record.

[10][13] Stewart played in the McDonald's All-American Game,[14] Jordan Brand Classic,[15] and Nike Hoop Summit.

[16] Stewart finished his high school career as a consensus five-star recruit and top-five player in the 2019 class.

[21] Sources: Heading into the season Stewart and teammate Jaden McDaniels were projected as potential top 3 picks for the 2020 NBA draft and possibly going first and second.

Stewart made his college debut for UW against the Baylor Bears in the 2019 Armed Forces Classic, recording 15 points and seven rebounds, including the game-winning basket in a 67–64 victory for Washington.

On November 22, 2020, Stewart, Trevor Ariza, and a conditional future first-round pick were traded to the Houston Rockets in exchange for Robert Covington.

[27] On November 21, 2021, Stewart was ejected in the third quarter after getting into a scuffle with LeBron James during a 116–121 loss to the Los Angeles Lakers.

[30] On March 9, 2023, the Pistons announced that Stewart was diagnosed with a left shoulder impingement and would be sidelined for at least three-to-four weeks.

On January 29, 2025, Stewart was ejected from the game against the Indiana Pacers with a flagrant 2 foul for shoving Thomas Bryant.