Allen was selected with the 22nd overall pick in the 2017 NBA draft by the Brooklyn Nets, where he quickly made an impact. In his rookie season, Allen made his NBA debut at just 19 years old and became the second-youngest player ever to set foot on court for the Nets. He finished the game with nine points on 3-for-3 shooting, two rebounds, and one block. Later that season, he joined the starting lineup for the first time and recorded a career-high 16 points and 12 rebounds in a win over the Philadelphia 76ers. He also scored a career-high 20 points in a game against the Los Angeles Lakers.

In 2018, Allen recorded a career-high 24 points and grabbed 11 rebounds in a loss to the Los Angeles Clippers. He also grabbed a season-high 14 rebounds and scored 13 points in a win over the Miami Heat. In 2019, he recorded 20 points and a career-high 24 rebounds in the Nets' overtime victory over the Houston Rockets.

Allen was traded to the Cleveland Cavaliers in January 2021 as part of a multi-player, four-team deal that sent James Harden to the Nets. He quickly established himself as a key player for the Cavaliers, averaging a career-best 16.1 points, 10.8 rebounds, 1.6 assists, 1.3 blocks, and .8 steals per game in the 2021-2022 season. Alongside rookie and fellow Cavalier Evan Mobley, Allen led Cleveland to a .537 winning percentage and the league's fifth-best defense for efficiency.

On October 20, 2021, in a game against the Memphis Grizzlies, Allen became the first player since the shot clock was introduced in the 1954-55 NBA season to make 10 or more field goals without a miss in a season opener. He was also named a replacement for James Harden on Team LeBron in the 2022 NBA All-Star Game.

In conclusion, Jarrett Allen's career highs showcase his impressive skills and potential as an NBA player. From his early days with the Brooklyn Nets to his current tenure with the Cleveland Cavaliers, Allen has proven that he can compete at the highest level and make a significant impact on the court. As he continues to grow and develop as a player, fans can expect to see even more impressive performances from this rising star.