Allen started the 2020-21 season with the Nets. He appeared in 12 games with Brooklyn, starting five contests. In 26.6 minutes per game, he averaged 11.2 points, 10.4 rebounds, 1.7 assists and 1.6 blocks. Allen was traded to Cleveland in January. He made his team debut Jan. 20 in a win over the Nets, where he posted 12 points, 11 rebounds, four blocks, two assists and one steal in 31 minutes. Overall, in the center's 51 appearances (40 starts) with the Cavaliers, he averaged 13.2 points, 9.9 rebounds, 1.7 assists and 1.4 blocks in 30.3 minutes. Between both teams, Allen recorded 25 double-doubles. In terms of NBA ranks, the fourth-year pro ranked 12th in total rebounds (631), 10th in total blocks (90) and sixth in field-goal percentage (61.8). His best performance of the season occurred as a member of the Cavaliers on Feb. 24. In the 112-96 win over the Rockets, Allen posted 26 points (10-11 FG, 1-1 3Pt, 5-8 FT), 18 rebounds, four blocks and two assists in 37 minutes. On April 25 against the Wizards, Allen scored a career-high 27 points.

The 2019-20 season marked Allen's third in the league and third with the Nets. He set new per-game career highs in points (11.1), rebounds (9.6), assists (1.6), steals (0.6) and field-goal percentage (64.9 percent). On Nov. 25, the Texas product delivered his best performance of the season in a two-point win over the Cavaliers. In 34 minutes, Allen recorded 22 points on 9-of-10 shooting with 21 rebounds, three assists, two steals and two blocks. That effort put him in great company, as he became one of only 40 players in the history of the Association to accumulate that statline. At season's end, Allen ranked third in the NBA in field-goal percentage, 10th in total rebounds (671), 13th in blocks (92), second in offensive rating (132.4) and 14th in win shares (8.2). Following the NBA's hiatus due to coronavirus, the Nets were one of 22 teams invited to resume the season in the Orlando bubble. Allen stepped up his game in his six of a possible eight appearances, averaging 15.7 points, 11.0 rebounds, 4.2 assists and 1.2 blocks in 34.1 minutes. His best performance in the bubble was in an eight-point win over the Wizards on Aug. 2 in which Allen generated 22 points (including 10-of-10 shooting from the line), 15 rebounds, two assists, one steal and one block in 38 minutes. On Aug. 7, during the NBA's restart in Orlando, Allen set a new career mark with eight assists in the 119-106 victory over the Kings. In that matchup, the center also posted 17 points, 11 rebounds and a block in 36 minutes. All in all, he totaled 30 double-doubles on the season. In four playoff games against the Raptors, Allen averaged 10.3 points, 14.8 rebounds, 2.3 assists and 1.8 blocks across 33.0 minutes.

Jarrett Allen has quickly developed a reputation as one of the NBA's most effective shot blockers through just two NBA seasons. During the 2018-19 season, he seized the starting center job in Brooklyn and helped Brooklyn make the postseason for the first time since 2015. Allen averaged 1.5 blocks per game in 2018-19 despite playing just 26 minutes per contest. He began the 2018-19 season with at least one rejection in each of Brooklyn's first seven games. Allen blocked at least three shots on a dozen different occasions, including a season-high six blocks against Cleveland on Feb. 13. The 22nd overall selection in 2017 also set career highs in points (10.9) and rebounds (8.4) per game and doubled his assists to 1.4 per contest. Allen does most of his offensive work around the rim. He finished the season shooting 59 percent from the field, a small improvement over his rookie season. Allen's Nets lost to the Sixers in the first round of the playoffs. Allen started all five playoff games for the Nets, averaging 11.0 points, 6.8 boards and 0.6 blocks per contest.

Allen made his NBA debut in the second game of the season, scoring nine points while shooting 3-for-3 from the field and grabbing two rebounds in a win over Orlando. The 22nd overall pick would appear in 72 games - making 31 starts - and average 8.2 points, 5.4 rebounds, and 1.2 blocks per game. He hit double-digits for the first time when he dropped 12 points along with six rebounds against Atlanta on Dec. 2. Allen would record four double-doubles on the year, the first coming in January when he notched 12 points and 11 rebounds versus Oklahoma City. He'd go on to score 13 points and a season-high 14 rebounds against Detroit on Feb. 7. Allen also dropped a peak of 20 points against the Lakers on Feb. 2. He recorded a season-high seven offensive rebounds during a 15-point, 13-rebound game against Sacramento on Mar. 1. He produced a high of five rejections against Milwaukee on Apr. 5. Allen shot .776 from the line, draining 114 of his 147 attempts. He'd also go on to lead the Nets with 144 offensive rebounds.