[2] Hennen was born in Phoenix, Arizona, and grew up in the San Francisco Bay Area, where he first became interested in racing motorcycles at the age of 15.
[2][4] After spending four years developing his riding skills in regional races, he entered his first professional event as a novice at the end of the 1971 season.
[4] In 1973 he competed in 250cc Junior Class road racing events riding a Suzuki X6 borrowed from Grant and modified by his brothers, Chip and David Hennen.
[6] Hennen scored the first major victory of his motorcycle racing career in 1974 at the Daytona International Speedway when he set the fastest qualifying time and won the Junior Class event aboard the TZ700.
[7] Hennen gained his first international exposure in 1974 when Grant took him to compete in the New Zealand Marlboro Series run over the Christmas/New Year period.
[2] The Transatlantic Trophy match races pitted the best British riders against the top American road racers on 750cc motorcycles in a six-race series during Easter weekend in England.
[2] Hennen showed promise in the AMA national championship by claiming the pole position at the Ontario Motor Speedway, but his motorcycle experienced mechanical issues and his best result of the season was a fifth place at Laguna Seca.
[2] The 1973 oil crisis severely impacted all forms of motorsports in the United States and Suzuki made the decision to withdraw their team from the AMA national championship at the end of the 1975 season, leaving Hennen without a job until Rod Coleman offered him a Suzuki TR750 race bike which he had salvaged after learning that the factory planned to discard it.
[4] Hennen rode the ex-factory motorcycle, built from discarded spare parts, to a dominating win at the 1976 New Zealand Marlboro Series, then scored an impressive third place at the 1976 Daytona 200 behind Johnny Cecotto and Gary Nixon, now riding for the Kawasaki factory racing team.
Hennen competed on a shoestring budget, driving to races across Europe in a small van with his brother Chip serving as his team manager as well as his mechanic.
[13] After a slow start to the 1976 season, Hennen finished runner-up in the Dutch TT at Assen, behind Sheene and ahead of multi-time World Champion Giacomo Agostini.
Having secured the championship, Sheene then chose not to compete in the final three rounds as he disliked riding the dangerous circuits remaining on the schedule.
[4][15] His upset victory was so unexpected that the Finnish race organizer did not have a copy of the American national anthem to play during the awards ceremony.
The Imatra Circuit, home of the Finnish Grand Prix, was notoriously dangerous featuring curbs, manhole covers, street signs and light poles along with a railroad crossing.
[16] The final race of the season was the German Grand Prix held at the daunting, 14.2 miles (22.9 km) long Nürburgring racetrack, considered too dangerous for the Formula One championship.
[1] Having established himself as one of the top three 500cc riders in 1976 earned Hennen a position on the Heron-Suzuki factory racing team for the 1977 season, with newly crowned 500cc World Champion Sheene as his teammate.
[24] Seeking a psychological advantage over his rival, Sheene publicly disparaged his American teammate, telling journalists, “If you pay peanuts, you get a monkey”.