Johnson was born in Wilkes County, North Carolina, the son of Lara Belle Money and Robert Glenn Johnson, Sr.[1] He grew up on a farm and, like many of the pioneers of stock car racing, developed his driving skills running moonshine as a young man. He consistently outran and outwitted local police and federal agents in auto chases, and he was never caught while delivering moonshine to customers. Johnson became something of a legend in the rural South, where his driving expertise and "outlaw" image was much admired. Johnson is credited with inventing the "bootleg turn," in which a driver escapes a pursuer by sharply putting his speeding car into a 180-degree turn on the highway, then speeding off in the opposite direction before his pursuer can turn around. Johnson was also known to buy and use police lights and sirens to fool policemen who had set up roadblocks into thinking that he was a fellow policeman; upon hearing his approach, the police would quickly remove the roadblocks, allowing Johnson to escape with his moonshine.

In 1955, Johnson decided to give up delivering moonshine for the more lucrative (and legal) career of being a NASCAR driver. He found that he was able to easily translate his "moonshiner" driving skills—hard-won on mountain roads—to the highly-pitched racing tracks of NASCAR. In his first full season, he won five races and finished sixth in the 1955 NASCAR Grand National points standings.

In 1956, federal tax agents found Johnson working at his father's moonshine still in Wilkes County and arrested him. Johnson was convicted of moonshining and was sent to the federal prison in Chillicothe, Ohio. He served 11 months of a two-year sentence.

Johnson returned to the NASCAR scene in 1958 and picked up where he left off, winning six races. In 1959, he won five more NASCAR Grand National races; by this time he was regarded as one of the best short-track racers in the sport.

His first win at a "superspeedway" came at the Daytona 500 in 1960. Johnson and his crew chief Ray Fox were practicing for the race, trying to figure out how to increase their speed, which was 22 miles per hour slower than the top cars in the race. During a test run a faster car passed Johnson. He noticed that when he moved behind the faster car his own speed increased due to the faster car's slipstream. Johnson was then able to stay close behind the faster car until the final lap of the test run, when he used the "slipstream" effect to slingshot past the other car. By using this technique Johnson went on to win the 1960 Daytona 500, despite the fact that his car was slower than others in the field. Johnson's technique was quickly adopted by other drivers, and his practice of "drafting" has become a common tactic in NASCAR races.[2]

In 1963 he had a two-lap lead in the World 600 at Charlotte before a spectator threw a bottle onto the track and caused Junior to crash; he suffered only minor injuries.

He retired in 1966. In his career, he claimed 50 victories as a driver, and 11 of these wins were at major speedway races. He retired as the winningest driver never to have a championship.