After the season, he entered the 2017 NBA draft but did not initially hire an agent before deciding to forgo his remaining three years of collegiate eligibility.

[12] He was 19 years 182 days when he made his NBA debut on October 20, becoming the second-youngest player ever to set foot on court for the Nets, just behind Derrick Favors.

[13] On January 25, 2018, Allen joined the starting lineup for the first time and recorded a career-high 16 points and 12 rebounds in a 116–108 win over the Philadelphia 76ers.

[26] On January 14, 2021, Allen was traded to the Cleveland Cavaliers in a multi-player, four-team deal with the Rockets that sent James Harden to the Nets.

[30] Allen was named a replacement for Harden, for whom he was traded a year prior, on Team LeBron in the 2022 NBA All-Star Game.

Alongside rookie and fellow Cavalier Evan Mobley, Allen led Cleveland from a .306 winning percentage and the league's sixth-worst defense to a .537 winning percentage and the league's fifth best defense for efficiency; it was .578 before Allen suffered a fractured finger injury in early March.

[33] The next day, Allen posted 30 points, 12 rebounds, six assists and two blocks on 15-of-17 shooting from the field in a 119–111 loss against the Milwaukee Bucks.

[34] On January 22, 2024, he set the franchise record for consecutive double-doubles during a win against the Orlando Magic by passing the 11-game streak held by both Andre Drummond and Elmore Smith.