After the Bucks had drafted in the NBA Draft in 1977 already with the first pick Kent Benson, they took only a few minutes later, Johnson in 3rd place. Under coach Don Nelson, Johnson became one of the best small forwards in the league. For his achievements in the first game year Johnson was appointed to the 1978 NBA All- Rookie Team. He is now regarded as a prototype of the Point Forward, who knew how to combine his size with skillful ball control and game overview. So he was called four times to the NBA All- Star Game (1979, 1980, 1981 and 1983 ) and once in the first All- NBA Team (1979) and twice in the second All - NBA Team ( 1980-81 ). He led the Bucks between 1980 and 1984 to four division titles.
1984 Johnson was transferred among others, Terry Cummings and Ricky Pierce to the Los Angeles Clippers. In the unsuccessful Clippers Johnson remained an important player. In 1986 he was appointed one last time into the All-Star Game. 1989 suffered a serious neck injury, the forward end the forced him to his career. Johnson returned in 1989 in the NBA back and played 10 games for the Golden State Warriors, but far below its level of performance during his time with the Bucks.
During his playing days Johnson scored 20.1 points, 7.0 rebounds and 3.5 assists per game.