French auto racing driver Henri Fournier introduced DeRosier to motorcycles in 1898 while visiting the United States.

DeRosier earned a strong reputation as a daring rider not only by winning races, but bouncing back from numerous injuries.

After winning top rider honors at the Federation of American Motorcyclists national championship in 1908 at Paterson, New Jersey, he attracted the attention of the Indian Motocycle Company.

Like the bicycles, the motorcycles raced on wooden velodromes, one-quarter to one-half mile (0.40 to 0.80 km) saucer shaped board track speedways.

Severely injured in a match race with a top California rider named Ed Lingenfelder, DeRosier hemorrhaged for a week.

[3] Redeeming himself, he defeated English Champion Charles Collier at the famous Brooklands oval speedway in a match race just a few days later.

At Brooklands on August 5, 1911 he broke the outright speed record (held by Henri Cissac since July 1905) when he covered a flying kilo in 25.2 seconds equal to 88.77 mph (142.86 km/h).