Hennen was born in Phoenix, Arizona, and grew up in the San Francisco Bay Area, where he developed an interest in motorcycle racing around the age of fifteen. He entered professional competition in 1971. His formative years as a racer were shaped by a relationship with Suzuki factory road racer Ron Grant, who served as his mentor and helped him obtain competitive machinery. Hennen focused on road racing rather than the dirt track discipline that many of his American contemporaries pursued.
In 1974, aboard a Yamaha TZ700 sourced through Grant, Hennen won the Junior Class event at Daytona and went on to claim the 1974 AMA Junior road racing championship. He also began competing in the New Zealand Marlboro Series over successive Christmas seasons, winning the event three years running from 1975 to 1977 and setting a lap record at the Cemetery Circuit in Whanganui that stood for six years.
Hennen entered the 500cc Grand Prix World Championship in 1976 as a privateer, competing with a production Suzuki RG500 funded through Rod Coleman, the Suzuki importer for New Zealand who had strongly advocated for him with the Suzuki factory. Running on a minimal budget, he and his brother Chip drove between European race venues in a small van. Despite the circumstances, Hennen finished runner-up at the Dutch TT at Assen in his first season, behind eventual champion Barry Sheene.
The defining moment of his career came at the 1976 Finnish Grand Prix at the notoriously dangerous Imatra street circuit. With Sheene absent after clinching the championship early, Hennen won the race, becoming the first American to win a 500cc Grand Prix. The result was unexpected enough that the Finnish organisers had no recording of the American national anthem to play at the ceremony. He ended that debut season third in the overall standings.
His performance earned him a place on the Heron-Suzuki factory team for 1977, riding alongside Sheene. The relationship between the two was tense, with Sheene consistently securing preferential equipment. Despite this, Hennen scored four podium finishes in 1977, including a victory at the British Grand Prix at Silverstone when both race leader Steve Parrish and second-placed John Williams crashed out on the final lap. He again finished the season ranked third in the world championship.
Hennen dominated the 1978 Transatlantic Match races, taking the top aggregate score, before the Grand Prix season opened. He won the Spanish Grand Prix at Jarama to take the championship lead, then continued trading positions with Sheene and the newly arrived Kenny Roberts. When the series reached its mid-season point, Hennen and Roberts were separated by a single championship point.
Hennen was sent to compete in the 1978 Isle of Man TT by the British-based Suzuki factory team despite most of the sport's leading riders boycotting the event due to its extreme danger. He had just completed the first ever sub-20-minute lap of the mountain circuit on a 500cc Suzuki when he crashed heavily at Bishopscourt on the final lap. A rumour circulated at the time that he had struck a bird, but this was never confirmed. The injuries Hennen sustained, including serious brain damage, ended his racing career permanently.
Hennen's recovery from the crash was slow and incomplete. He retained impairments to speech, memory, and mobility for the rest of his life, but was able to maintain a degree of independence in the San Francisco Bay Area. He became a practising Christian and worked in go-kart manufacturing, jet engine maintenance for United Airlines, and as an application engineer for a motorcycle parts distributor. He was inducted into the AMA Motorcycle Hall of Fame in 2007.
Although Hennen never won a world championship, his career is historically significant as a precursor to the era of American dominance in 500cc Grand Prix racing. Riders such as Kenny Roberts, Freddie Spencer, Eddie Lawson, Wayne Rainey, Kevin Schwantz, and Mick Doohan — who would collectively dominate the championship through the 1980s and into the 1990s — came from the same American and Australian dirt-track racing tradition that had shaped Hennen. His victory at the 1976 Finnish Grand Prix opened the door through which an entire generation of American world champions would follow. Hennen died on April 6, 2024, at the age of seventy.