Spencer went on to win the World Championship in 1983 in the toughest competition on record: He and Kenny Roberts split 12 wins and 12 pole positions between them, with Spencer taking the win by a scant 2 points in the final race of the season.

At 21, Fast Freddie Spencer had become the youngest World Grand Prix Champion in history. 1984 was a hallmark year for Freddie Spencer. He played a major role in the research and development of a new two-stroke Grand Prix machine for Honda, considered the most radically designed bike in post-war Grand Prix racing. Due to various teething problems, Spencer started just five races on the new machine, but won four of those and still finished fourth in the World Championship point standings.

In 1985, Fast Freddie Spencer was back with a vengeance. The young man who had taken the championship from King Kenny Roberts in 1983 was soon to become the first person in history to win both the 250cc and 500cc World Championships in the same season. In the 250cc classification, Freddie competed in ten events, qualified on the pole six times, and won seven races. In the eleven 500cc races, Freddie qualified first nine times and won seven. This achievement stands alone in the history of modern Grand Prix racing; no competitor today would even attempt such a feat, yet Spencer accomplished it while setting nine new track records. Even more incredibly, in the same year, he won all three major divisions (250cc, 500cc and Superbike) at the AMA National at Daytona International Speedway. He is the first and only competitor ever to do so.