Johnson was born in El Cajon, California where his father was an avid motorcyclist who bought his son a mini-bike when he was 3 years old. When he turned 16 in 1980, he earned his pro license. He won his first 250cc class national championship in 1984 for the Yamaha factory motocross team. For the 1986 season, he was offered a job with the Honda team by team manager and five time former world champion Roger De Coster. He battled his Honda teammate David Bailey throughout the 1986 season, coming away with the 250 title and the Supercross crown. De Coster picked Johnson, Bailey and another Honda teammate Johnny O'mara to represent the US in the Motocross des Nations in Maggiora, Italy. Team U.S.A. won with a clean sweep. Back home, he finished second to Bailey in the 500 class. The rivalry was short-lived as just prior to the start of the 1987 season Bailey was paralyzed in a practice crash.