Hepburn was born in Somerville, Massachusetts. His family moved to Los Angeles, California, when he was 10. He began riding motorcycles as a delivery boy in Los Angeles just after the turn of the century. He was a big fan of the popular board track races of the day. Hepburn took an offer for a delivery job that paid him a weekly oil and gas allowance. Hepburn used some of his allotment to begin racing the boards in the amateur ranks.
To give amateur riders a little money, a hat was passed through the crowd. A glimpse of Hepburn's kind character came through when, in an amateur race, he'd won 30 cents. One of his fellow competitors had fallen during the race, so Hepburn took the 30 cents and bought his fellow competitor a box of malted milk balls to cheer him up.
When he was 18 years old, Hepburn took an offer to travel with a barn-storming group that rode the motordromes (board tracks) and a few dirt tracks across the Midwest. In 1916, Hepburn bought an ex-factory Excelsior and started earning podium finishes at the nationals.
World War I put a hold on Hepburn's racing. He signed up to be a pilot and was just three weeks from earning his flight certificate when the war came to an end.
In June of 1919, Hepburn secured a factory ride with Harley-Davidson. He became part of what may have been the greatest factory racing team of all time. Hepburn's Harley teammates included Ray Weishaar, Albert "Shrimp" Burns, Maldwyn Jones, Red Parkhurst, Fred Ludlow and Otto Walker, all of whom would become Motorcycle Hall of Fame inductees. He won the M&ATA 200 Mile National Championship at Ascot Park in Los Angeles on June 22, due in large part to his pit crew, which was directed by Dudley Perkins. The crew managed incredibly fast 38-second pit stops for gas, oil and new rear tire.
On July 4, 1921, Hepburn won the Dodge City 300, in perhaps the greatest performance of his career. It was the last running of that classic event and all of the factory teams and riders were on hand. Hepburn took the lead early in the race and was only passed briefly while making a pit stop. He went on to win the race by 12 minutes over second-place rider Johnny Seymour. Hepburn broke all existing 300-mile records in that race.
The next year he signed with Indian. Quite by accident, Hepburn got to try his hand at sidecar racing in Milwaukee. Floyd Dreyer had crashed in a heat race and was sent to the hospital. Dreyer's crew got the rig straightened out and gave Hepburn a chance to drive. He hopped on and proceeded to win one of the 1922 sidecar national titles. That year he also defended his 300-Mile National title. The race had moved from Dodge City to Wichita and was not backed by the factories. The promoters had to pay appearance money to get the top stars of the day to show up. Still, Hepburn turned in another great ride and won by 18 minutes.
In 1923, Hepburn won the Pacific Coast Championship. In 1924, he returned to Harley-Davidson and was runner-up to Jim Davis at the national finals in Syracuse.
At the end of the 1924 season, Hepburn spent his honeymoon with wife, "Sparky," by racing in Australia with Jim Davis, Johnny Seymour and Paul Anderson. While "down under," Hepburn set a new speed record riding a Harley-Davidson on Adelaide Beach. On his return from Australia, he retired from professional motorcycle racing to take up auto racing.
In 1925, he competed in a car built by Harry Arminius Miller in the first of fifteen appearances at the Indianapolis 500. In 1929 he qualified third but gear problems resulted in him having to drop out after only fourteen laps. He finished third in 1931 and although he led the famous race three times in three different decades and had four "Top 5" finishes, his best result came in 1937 with a second place finish 2.16 seconds behind Wilbur Shaw, the closest ever at the time. Hepburn set the Indianapolis track record in qualifying for the 1946 race, but didn't start on the front row since he was not a first weekend qualifier.
For a time, Hepburn served as president of the American Society of Professional Automobile Racers.
In 1946, at the age of fifty, Hepburn drove a "Novi Governor SPL," setting a one-lap record of 134.449 mph in qualifying and led forty-four laps in the race before the car stalled out on the 121st lap.
Hepburn died in qualifying for the Indianapolis 500 on May 16, 1948.