Roberts grew up in Modesto, California, and began motorcycle racing as a teenager after a friend dared him to ride a minibike. He built his own first motorcycle from a lawn mower engine mounted to a bicycle frame. Sponsored by a local Suzuki dealer from 1968, he dropped out of high school before his senior year to pursue racing professionally. Yamaha USA signed him at age 19, pairing him with former 250cc world champion Kel Carruthers as mentor and crew chief — a partnership that would prove defining for both men.
Roberts won the AMA Grand National Championship in 1973 and 1974, becoming only the second rider after Dick Mann to accomplish the Grand Slam — victories at a mile, half-mile, short-track, TT Steeplechase, and road race in a single season. At the 1975 Indianapolis Mile, he famously mounted a Yamaha TZ750 two-stroke road racing engine in a dirt track frame, charging from last on the grid to win the race, later quipping "They don't pay me enough to ride that thing." The AMA subsequently banned two-stroke engines in dirt track competition.
His European exposure came in stages: a third place at the 1974 Dutch TT on his world championship debut in the 250cc class, victories in the Transatlantic Trophy match races against British riders, and wins at the prestigious Imola 200 in Italy. European fans and journalists were struck by his bucking, sliding riding style — Italian press dubbed him "Il Marciano," the Martian.
When Yamaha could not build a competitive dirt tracker, they sent Roberts to Europe for the 1978 500cc World Championship campaign. He arrived as an unknown quantity to many in the paddock; Barry Sheene, the defending double champion, publicly dismissed him as no threat.
Roberts won his first 500cc Grand Prix in Austria, then followed with victories in France and Italy. After weathering a rain-soaked British Grand Prix disputed over scoring, he arrived at the season-ending German Grand Prix at the Nürburgring needing a top-four finish ahead of Sheene. He qualified second and finished third, claiming the title — the first world championship won by an American in Grand Prix road racing history.
The 1979 title defence nearly ended before it started when Roberts suffered back injuries and a ruptured spleen in a pre-season testing crash in Japan. He recovered to win three rounds including the British Grand Prix at Silverstone, where he beat Sheene by three-hundredths of a second in a race now considered one of the greatest motorcycle races of the 1970s. A third place in the French finale, combined with a crash by rival Virginio Ferrari, secured his second consecutive title.
In 1980, with Yamaha elevating his operation to de facto factory status, Roberts won the opening three races of the season and built a sufficient points cushion to withstand a Suzuki resurgence in the second half of the year, claiming his third 500cc world championship.
His 1983 farewell season produced what many regard as one of the greatest championship battles in motorcycle Grand Prix history, fought against Freddie Spencer on the Honda NS500. Spencer won the opening three rounds and led by points at mid-season, but Roberts won three consecutive races — Netherlands, Belgium, and Britain — to draw level. The Swedish Grand Prix decided it: Spencer passed Roberts in the closing stages using a move Roberts considered reckless, taking a narrow victory. Roberts won the final round at San Marino but Spencer secured second place, claiming the title by a slim margin. The two riders each won six of the season's twelve races.
In earlier seasons, Roberts' battles with Sheene had also defined the era. At the 1979 Spanish Grand Prix, Roberts — angered by promoters refusing to pay guaranteed starting money — won the race and then refused to accept the winner's trophy on the podium, telling organizers to melt it and sell it to cover competitor expenses. The FIM initially suspended him; the suspension was reduced to probation. The gesture resonated across the paddock.
Roberts was the most prominent rider advocate of his generation. He repeatedly challenged the FIM over dangerous circuit conditions, most notably instigating a riders' boycott at the 1979 Belgian Grand Prix at Spa after a freshly paved surface produced diesel seepage that many riders considered life-threatening. In December 1979, Roberts and other top riders announced plans for a rival racing series called the World Series, threatening to break from FIM control entirely. Though the competing series never materialized due to difficulties securing venues, the threat forced the FIM to substantially increase prize money — by up to 500% — and impose stricter safety regulations on circuit operators. Roberts had arrived in an era when world champions raced at venues with railroad crossings and hay bales around telephone poles for prize money as low as $200.
After retiring from riding following selected 1984 events, Roberts formed a team that became the official Yamaha factory operation in 1990. Wayne Rainey won three consecutive 500cc world championships under his management. Following Rainey's career-ending crash at the 1993 Italian Grand Prix, the team continued with Luca Cadalora. In 1997 Roberts left Yamaha after more than 25 years to build his own three-cylinder motorcycle with engineering assistance from Tom Walkinshaw Racing. The project eventually raced as Team Roberts through the MotoGP era until sponsorship dried up after 2007.
Roberts' riding technique — forcing the rear wheel to break traction to steer through corners — became the universal Grand Prix method. From 1983 to 1999, every 500cc world championship was won by a rider with a dirt track background. His habit of extending his knee to skim the track surface, using duct tape as makeshift protection, led directly to the purpose-built knee pucks now used by all motorcycle road racers. He was named a Grand Prix Legend by the FIM in 2000. His son Kenny Roberts Jr. won the 2000 500cc World Championship, making them the only father and son to each hold the title.