Bell committed to the University of Oregon to play college basketball. Bell redshirted in 2013. As a redshirt freshman at Oregon, Bell averaged 5.1 points, 6.1 rebounds and 2.7 blocks per game. His 94 blocks were a school record for a season. As a redshirt sophomore, he averaged 7.0 points, 5.4 rebounds and 1.8 blocks per game. As a redshirt junior, Bell averaged 10.9 points, 8.8 rebounds and 2.3 blocks per game and helped lead Oregon to the Final Four.
On April 18, 2017, Bell declared for the 2017 NBA draft, and was picked with the 38th pick by the Chicago Bulls and later traded to the Golden State Warriors for $3.5 million in cash considerations. Bell participated in the 2017 NBA Summer League. On October 23, 2017, Bell dunked the ball after bouncing it off the backboard to himself during at the end of a 133–103 rout over the Dallas Mavericks. The play angered the Mavericks' players and coaches and ignited a debate on social media. On December 1, 2017, Bell scored 16 points on 8-for-10 shooting in 16 minutes of playing time against the Orlando Magic in a 133–112 victory. On December 22, against the Los Angeles Lakers, he scored a career-high 20 points on 9-for-13 shooting to go with his career-high 10 rebounds in 25 minutes. On April 14, 2018, Bell made his debut in the NBA playoffs, coming off of the bench with three points and two rebounds in a 113–92 win over the San Antonio Spurs. The Warriors made it to the 2018 NBA Finals where they would sweep the Cleveland Cavaliers in four games. Bell played in the 2018 NBA Summer League for the Warriors. On March 27, 2019, the Warriors suspended Bell for one game due to "conduct detrimental to the team". On May 16, 2019, Bell came off the bench and scored a playoff career high 11 points to go with 3 rebounds, 2 steals, and an assist and a block apiece in a 114 to 111 win. On May 20, Bell made his first career playoff start in a series clinching win against the Portland Trail Blazers, contributing 7 points, 2 assists, and a steal. The Warriors reached the 2019 NBA Finals, but were defeated in 6 games by the Toronto Raptors.
On February 5, 2020, Bell and Robert Covington were traded from the Timberwolves to the Houston Rockets in a four-team, 12-player deal. The next day, he was traded to the Memphis Grizzlies for Bruno Caboclo and a 2023 second-round pick swap. On March 2, 2020, Bell was waived by the Memphis Grizzlies.
