Johnson was selected third overall in the 1977 NBA Draft by the Milwaukee Bucks, who were coached by Don Nelson. Johnson helped lead Milwaukee to several division titles (1980, 1981, 1982, 1983, 1984). In his second season in 1978–79, he was the NBA's third leading scorer (25.6 PPG), behind George Gervin (29.6) and Lloyd Free (28.8). Johnson claims to have coined the term "point forward", a position he played out of necessity in 1984. During the 1984 playoffs, Milwaukee became short on point guards due to injuries. Nelson instructed Johnson to set up the offense from his forward position. Johnson responded, "OK, so instead of a point guard, I'm a point forward". However, Del Harris, then-Bucks assistant coach, claims that he first mentioned the term to Nelson. Johnson and the Bucks reached as far as the Eastern Conference Finals twice, in 1983 and again in 1984.
In the 1984 offseason, Nelson — who was also Bucks general manager — traded Johnson, forward-guard Junior Bridgeman, forward Harvey Catchings and cash to the Los Angeles Clippers in exchange for forward Terry Cummings, and guards Craig Hodges and Ricky Pierce. This was a homecoming for Johnson, as he grew up and attended high school just a few miles from the Clippers' home at Los Angeles Memorial Sports Arena. The Clippers struggled to win. Johnson later said that playing for those losing Clippers teams "kind of wore you down and made you feel like you were kind of the JV team in Los Angeles." Being named the team captain by head coach Don Chaney, a fellow Louisianan, was one of the few things that kept him from demanding a trade. During a game in the 1986–87 season, Johnson suffered a neck injury, which effectively ended his career. Johnson made a brief comeback during the 1989–90 season, playing only 10 games with the Warriors before retiring on December 27, 1989.
He has five sons, Kris, Josiah, Joshua, Moriah and Cyrus. Kris, like his father, played basketball at Crenshaw High and UCLA. Johnson and Kris are the first father–son combo to be honored as Los Angeles City Section 4-A Player of the Year.[note 1] They are also one of four father-son duos to each win an NCAA basketball championship and the only ones to accomplish it at the same school.[note 2] Josiah also played basketball at UCLA. Moriah is a star on the BET's Baldwin Hills.