Powell was a two-time all-state high school player in California and led his team to a state championship before joining UCLA.

After entering his senior year in 2014–15 as the Bruins' top returning player, he emerged as a team leader and received first-team All-Pac-12 honors.

[4] He helped rebuild their basketball program,[4] leading the team to consecutive California Interscholastic Federation (CIF) San Diego Section 2A championships in 2009 and 2010.

The team also captured the 2010 CIF Division II Boys' State Championship, with Powell scoring 24 points in a 74–59 win over Mountain View St. Francis.

He finished his junior season averaging 19.7 points, 4.9 rebounds, and 2.7 steals, and earned first-team all-state as well as Western League Most Valuable Player (MVP) honors.

[1] He finished his high school career winning the San Diego County All Star Game Dunk Contest.

[8] While he excelled as a driving guard in high school, his role transformed at UCLA, taking less than one-fourth of his shots around the rim and close to half from outside on three-point attempts,[9] connecting on 38.5% over the last 18 games.

[10] He considered transferring to San Diego State after the season,[11] but decided to stay primarily because UCLA coach Ben Howland was fired.

[10] Powell credited Howland for his development on defense, but he stated that "there were a lot of things going on that weren’t fair for me" and "I definitely wanted to see a change.

[11][15] UCLA opened the 2014 NCAA tournament in San Diego, enabling him to play in his hometown for the first time since high school.

[22][23] Although freshman teammate Kevon Looney was seen as a potential NBA lottery pick, Powell became the Bruins' most consistent performer.

[33] With Carroll out after knee surgery and James Johnson sidelined with a sprained ankle, Powell made his first NBA start on February 2, 2016, going scoreless with two steals in 15 minutes in a 104–97 win over the Phoenix Suns.

[36] Two days later, he scored 10 points in a 105–97 win over the Atlanta Hawks, helping the Raptors record a 50-win season for the first time in franchise history.

[37] On April 8, with All-Stars DeMar DeRozan and Kyle Lowry resting, Powell set a new season high with 27 points in a 111–98 win over the Indiana Pacers.

With DeMarre Carroll sitting out back-to-back games early on in the season, Powell received a number of starting assignments.

[62] Toronto advanced in the playoffs to the conference semifinals against the Boston Celtics, which Powell helped tie the series at 3 after scoring 15 of his 23 points in the two overtime periods in a 125–122 win in Game 6.

[64] On March 25, 2021, the Portland Trail Blazers acquired Powell from Toronto in exchange for Gary Trent Jr. and Rodney Hood.

At the time, Powell had made 31 starts in his 41 games and was averaging a career-high 19.6 points, while his 3-point field goal percentage of 43.9% ranked 10th in the league.

[68] On February 4, 2022, Powell was traded, alongside Robert Covington, to the Los Angeles Clippers in exchange for Eric Bledsoe, Justise Winslow, Keon Johnson, and a 2025 second-round pick.