The coronavirus-shortened 2019-20 season was Harrison's third season in the Association. He appeared in 43 out of 65 possible games for the Bulls during the shortened season, with 10 starts. Harrison averaged 4.9 points, 2.0 rebounds and 1.1 assists over 11.3 minutes per contest. The 6-foot-7 wing shot 46.7 percent from the field and produced career highs in both three-point percentage (38.1) and free-throw percentage (78.0). On Nov. 20, Harrison set a new career high with 11 rebounds, plus added 15 points, five assists and three steals, during a win over the Pistons. On Mar. 6 against the Pacers, Harrison set new career highs in both points (25) and made threes (five) during 34 minutes of play. In that game, he also contributed four rebounds, four assists, three steals and two blocks. Harrison finished the season with eight double-digit scoring games, four games with at least five rebounds and eight games with at least three assists. In the eight games in which Harrison played 20-plus minutes, he averaged 11.9 points, 5.6 rebounds, 3.1 assists, 2.1 steals and 1.5 blocks. Harrison started the last six games for the Bulls before the March sports stoppage.

After being waived by the Suns, Harrison signed a deal with the Bulls in the 2018 offseason. Harrison found a new home in Chicago, averaging 19.6 minutes per contest as a key piece of the rotation. He played in 73 games, starting 11. Harrison made 43.2 percent of his field goals, 27.0 percent of his threes, and 66.7 percent of his free throws. All told, the sophomore averaged 6.5 points per game. He was pesky on defense, averaging 1.2 steals per contest. Late in the season, Harrison played a more sizable role with the Bulls. Over his last 11 games, he averaged 31 minutes per contest, generating 12.2 points and 5.9 rebounds per game. That stretch included his top game of the year, a Mar. 27 gem versus Portland. The 6-foot-4 guard produced 21 points on 10-of-17 shooting and 10 rebounds for his second career double-double. Two games later, he recorded another 10-rebound effort.

After spending the entire 2016-17 season with the G League's Northern Arizona Suns, Harrison split time between the G League and the NBA in 2017-18. He appeared in 23 games for Phoenix and 36 games in the minors. The 6-foot-7 guard averaged 11.0 points, 4.7 rebounds, 3.5 assists and 1.9 steals on 46.2 percent shooting for Northern Arizona. With the big club, he averaged 6.6 points, 2.7 rebounds and 2.4 assists. Harrison played 16.7 minutes per game and shot 47.6 percent from the floor. In his NBA debut, he collected four steals and four assists in under 14 minutes. His first career game with double-digit points came Mar. 10, when he dropped 14 against the Hornets. All told, Harrison posted six double-digit scoring games, including four in the final seven games. During that seven-game stretch, he averaged 10.4 points and 4.3 assists on 50.8 percent shooting. In the final game of the regular season, the Tulsa product recorded his first double-double, scoring a career-high 18 points with 10 assists against the Mavericks.