Roberts was born in Modesto, California. He rode his first motorcycle at age 12 on a dare, soon building his own machine from a lawnmower engine and a bicycle frame. After watching a local race in Modesto he committed to competition, and a Suzuki dealer named Bud Aksland spotted his talent in 1968 and offered sponsorship. Roberts dropped out of high school before his senior year to pursue racing full-time, entering his first professional race at San Francisco's Cow Palace the day after his eighteenth birthday.
His early career was managed by airline pilot Jim Doyle, who connected Roberts with Yamaha's American racing program and former 250 cc world champion Kel Carruthers, who became his long-time crew chief and mentor.
Roberts won his first AMA Grand National Championship in 1973, only his second expert season, amassing a then-record 2,014 points across 25 races on an underpowered Yamaha against dominant Harley-Davidson factory machinery. He claimed a second title in 1974 and became only the second AMA rider after Dick Mann to complete the Grand Slam — victories in all five disciplines: mile, half-mile, short-track, TT Steeplechase, and road race — within a single season.
A pivotal technical influence came in 1972 when Roberts observed Finnish rider Jarno Saarinen shift his body weight to the inside of corners. Roberts adopted and exaggerated the technique, extending his knee until it skimmed the track surface. That style, rooted in dirt-track instinct, became universal in motorcycle road racing.
At the 1975 Indy Mile, Roberts and Carruthers fitted a 170 mph Yamaha TZ750 two-stroke road-racing engine into a dirt-track frame. Starting last after struggling in qualifying, Roberts carved through the field to win by inches, causing the AMA to ban two-stroke engines from dirt-track competition in 1976.
When Yamaha USA could no longer develop a competitive dirt-track machine, Roberts went to Europe in 1978 to contest the 500 cc World Championship. Few gave him any chance; reigning champion Barry Sheene dismissed him as "no threat." Roberts proved otherwise. He won four Grands Prix — Austria, France, Italy, and a rain-controversied British Grand Prix at Silverstone — and clinched the title at the season-ending German Grand Prix at the Nürburgring, finishing third ahead of Sheene in fourth.
His riding style — braking early, then hard on the throttle so the rear tire spun and slid through corners — had never been seen on European circuits and unsettled rivals who relied on late-braking smooth arcs. Italian journalists called him "Il Marciano" (the Martian).
In 1979, Roberts suffered career-threatening back injuries and a ruptured spleen in a pre-season test crash in Japan. He recovered to win the second round in Austria and secured his second title, including a legendary battle with Sheene at the British Grand Prix at Silverstone won by three-hundredths of a second. That same year Roberts publicly threatened to create a rival race series called the World Series, forcing the FIM to raise prize money by as much as 500 percent and impose stricter safety requirements on promoters.
He secured his third consecutive 500 cc title in 1980 after Yamaha made the Yamaha USA squad their de facto factory team. The championship came despite a late-season Suzuki resurgence, Roberts having built a sufficient points lead before mechanical failures intervened.
Roberts announced 1983 as his final Grand Prix year, with Eddie Lawson joining as his Yamaha teammate. The season became one of the most celebrated in motorcycle racing history, Roberts and Honda's Freddie Spencer splitting twelve victories evenly. Roberts won the German, Austrian, Dutch, Belgian, and British Grands Prix, but Spencer clung close. At the penultimate round in Sweden, Spencer made a brave inside pass in the final corner — Roberts considered it foolish and dangerous — to take a crucial victory. Roberts won the finale in San Marino, but Spencer's second place clinched the championship by a slender margin.
Roberts continued to compete selectively in 1984, winning the Daytona 200 for the second consecutive year. He finished with 24 Grand Prix victories and 32 Grand National wins across his career.
After retiring as a rider, Roberts formed a team that fielded Wayne Rainey, who won three consecutive 500 cc world championships from 1990 to 1992. In 1997 Roberts left Yamaha after more than 25 years to build his own motorcycle, the three-cylinder KR3, using engineering support from Tom Walkinshaw Racing. The project expanded into MotoGP as the KR5 five-cylinder machine funded by Proton of Malaysia, and later the KR211V and KR212V using Honda engines. Kenny Roberts Jr. rode the KR211V to sixth in the 2006 MotoGP championship, including two podium results. The team withdrew after 2007 due to lack of sponsorship.
Roberts' influence on motorcycle racing has no modern parallel among American riders. His dirt-track cornering method — knee extended to the tarmac — evolved into the knee-puck and body-hanging style used by every road racer today. His political confrontations with race promoters and the FIM over safety and prize money transformed the professional environment of Grand Prix racing. Every 500 cc world champion from 1983 to 1999 came from a dirt-track background, a direct reflection of the style Roberts introduced.
He and his son Kenny Roberts Jr. — the 2000 500 cc world champion — remain the only father-and-son pair to have each won a premier-class motorcycle racing world title.
Roberts was inducted into the Motorsports Hall of Fame of America in 1990, the International Motorsports Hall of Fame in 1992, and the AMA Motorcycle Hall of Fame in 1998. The FIM named him a Grand Prix Legend in 2000.