Allen was selected by the Brooklyn Nets with the 22nd overall pick in the first round of the 2017 NBA Draft. He quickly established himself as a promising young center, known for his defensive prowess, shot-blocking ability, and athleticism. After several seasons with the Nets, Allen was traded to the Cleveland Cavaliers in January 2021 as part of a multi-team deal. With the Cavaliers, he has continued to develop his game and has become a key player for the team.