The University of Pittsburgh initially refused to allow his release to their in-conference rival Tar Heels but eventually relented.

Unhindered by injury, Johnson was the leading scorer for the Tar Heels with 16.9 points per game and averaged 5.8 rebounds and 2.4 assists.

Playing against Wake Forest in Winston-Salem, Johnson shot 79% from the field for the game and made his first six three-point attempts, ultimately finishing with 27 points in a 95–57 rout of the Demon Deacons.

[8] Against rival Duke in Cameron Indoor Stadium, Johnson finished with 26 points in an 88–72 victory against the Blue Devils, then the nation's top-ranked team.

[14] On October 26, 2019, Johnson made his NBA debut coming off the bench in a 130–122 win over the Los Angeles Clippers.

[18] After originally being out for the rest of March due to illness, Johnson returned to action as a starter on July 31, scoring 12 points in a 125–112 win over the Washington Wizards.

[20] He later recorded his second double-double of his rookie season on August 6, matching his season-high 12 rebounds with 14 points scored in a 114–99 win over the Indiana Pacers.

[21] Two days later, Johnson broke the franchise record previously set by Wesley Person for the quickest player to score at least 100 three-pointers in a 119–112 win over the Miami Heat, reaching it in only 54 games played.

[22][23] With Johnson starting for Phoenix, the Suns would have an 8–0 record in the 2020 NBA Bubble as of August 13, having a winning streak of at least 7 games for the first time since March 2010.

[27] He finished third in NBA Sixth Man of the Year voting, behind winner Tyler Herro and Kevin Love.

[31] Johnson made his Nets debut two days later, recording 12 points, seven rebounds, seven assists and three steals in a 101–98 loss to the Philadelphia 76ers.

[33] On November 19, 2024, Johnson had his best career game with the Nets, scoring 34 points and hitting 6 threes, leading them to a 1-point win over the Charlotte Hornets.