Roberts showed a natural talent for dirt track racing and began winning local races. In 1968, his race results drew the attention of a local Suzuki dealer Bud Aksland, who offered to sponsor Roberts aboard a Suzuki motorcycle. He made the decision to drop out of high school before his senior year to pursue a career in motorcycle racing. Roberts was allowed to compete professionally when he turned 18, and on the day after his eighteenth birthday, he entered his first professional race at San Francisco's Cow Palace, finishing in fourth place.
The motorcycle Technology of the late 1970s featured engines with power in excess of what the frames and tires of the day could accommodate. Roberts' riding style, bred on the dirt tracks of America, revolutionized road racing. Prior to his arrival in Europe, riders focused on attaining high entry speeds into corners, leaving braking until the last possible moment then, carving graceful arcs through the corners with both wheels in line. Roberts did just the opposite, braking early then, quickly applying the throttle which resulted in the rear tire breaking traction and spinning. The resulting tire spin caused the motorcycle to buck and shake as it continually lost then regained traction, creating a brutal, violent riding style that no one had ever seen before on the racetracks of Europe. His riding style was reminiscent of dirt track riding, where sliding the rear tire to one side is used as a method to steer the motorcycle around a corner. Because of his early application of the throttle, he was able to attain top speed faster than his competitors.
In 1971, Roberts won the AMA Rookie of the Year Award. In his first professional race as an expert class rider in 1972, Roberts rode to victory at the Grand National short-track race in the Houston Astrodome. Roberts made a name for himself that year by battling the dominant Harley-Davidson factory dirt track team aboard an underpowered Yamaha XS 650 motorcycle, making up for his lack of horsepower with sheer determination. He finished the season ranked fourth in the country. In 1973, in just his second season as an expert, Roberts won the national championship, amassing a record 2,014 points in the 25-race series.
Roberts returned to compete in the 1974 Grand National championship and won his first national road race at Road Atlanta on June 2, 1974. On August 18, Roberts won the Peoria TT race to complete a Grand Slam with victories in each of the five different events on the Grand National calendar. He claimed his second consecutive Grand National championship, winning six races and surpassing his 1973 points record by scoring 2,286 points in the 23 race series, collecting points in all 23 races. Roberts also entered his first world championship road racing event, winning the pole position before finishing third in the 1974 250 cc Dutch TT.
Roberts continued his road racing successes in 1975, winning three out of four races in the 1975 Transatlantic Match races. After having won the national championship in 1974, Roberts faced an increasingly difficult battle in dirt track races, as Harley-Davidson continued to improve their XR-750 dirt tracker while Yamaha struggled to maintain the pace. Roberts made up for his bike's lack of power with an almost fearless, determined riding style. He battled Harley-Davidson factory rider Gary Scott throughout the 1975 season but mechanical breakdowns hampered his title defense. He had been leading the Daytona 200 when mechanical problems yielded the victory to his Yamaha teammate Gene Romero. At the Ascot TT, Roberts battled from 17th place to take the lead before a broken sprocket ended his race. Roberts' fearless riding style was highlighted at the Indy Mile Grand National. In a desperate effort to keep Scott within reach in the points chase, Yamaha wedged a Yamaha TZ750 two-stroke road racing engine inside a dirt track frame. On a bike that was considered unrideable due to its excessive horsepower, Roberts came from behind on the two-stroke, and overtook the factory Harley-Davidson duo of Corky Keener and Jay Springsteen on the last lap for one of the most famous wins in American dirt track racing history. Afterward, Roberts was famously quoted as saying, "They don't pay me enough to ride that thing". Despite accomplishing another Grand Slam, this time in only one season, Roberts lost his crown, finishing second to Gary Scott in the 1975 national championship.
Although Roberts won four Grand Nationals in 1976, he continued to experience mechanical misfortunes as well as a horsepower deficit to the Harley-Davidson motorcycles in the mile and half-mile dirt track events. He had been leading the Daytona 200 once again when tire troubles forced him to make a lengthy pit stop, and Johnny Cecotto went on to win the race. He dropped to third in the national championship as Jay Springsteen claimed the title for the Harley-Davidson team. He returned to England in April 1977, winning four out of six races at the 1977 Transatlantic Match races. Roberts then travelled to Italy where he raced in the Imola 200, leaving no doubt he was capable of competing at the international level by winning both legs and setting a new track record. He returned to the United States to compete in the Grand National championship where he won five of the six road races that made up the pavement portion of the series. In the road race event at Sears Point, Roberts started the race at the back of the pack and passed the entire field within four laps to win the race. Despite being in contention for much of the season, Roberts was unable to win any of the dirt track events and eventually finished the year in fourth place.
Although Roberts won the 250 cc Grand Prix, Sheene claimed the victory in the 500 cc Venezuelan Grand Prix while Roberts' Yamaha suffered a mechanical failure on the starting line. In the second round at the Spanish Grand Prix, Roberts was leading the race by eight seconds when his throttle stuck, forcing him to settle for second place behind fellow American Pat Hennen. Roberts then won his first-ever 500 cc Grand Prix with a win in Austria, quickly followed by two more victories in France and Italy, along with two second-place finishes in the Netherlands and Belgium. At the 1978 Swedish Grand Prix, Roberts crashed during practice for the 250 cc race, sustaining a concussion and a thumb injury. Shaken up by the accident, he could do no better than seventh place in the 500 cc race. Sheene had come down with a debilitating virus at the Venezuelan round, but a string of podium finishes and a victory at the Swedish Grand Prix combined with Roberts' failure to score any points in the Finnish Grand Prix, allowed him to close the points gap.
In December 1979, Roberts made good on his threats when he, along with the other top world championship riders, released a letter to the press announcing their intention to break away from the FIM and create a rival race series called the World Series. When Roberts first arrived on the Grand Prix scene, motorcycle racers were competing for as little prize money as $200, at venues such as Imatra in Finland that featured railroad crossings and hay bales wrapped around telephone poles. In 1956, the reigning 500 cc world champion, Geoff Duke and thirteen other riders were given six-month suspensions for merely threatening to strike. Roberts adopted a confrontational, sometimes belligerent stance with race promoters, challenging the previously accepted poor treatment that motorcycle racers of the day were accustomed to receiving. Although the competing series failed to take off due to difficulties in securing enough venues, it forced the FIM to take the riders' demands seriously and make changes regarding their safety. During the 1979 FIM Congress, new rules were passed increasing prize money substantially and in subsequent years, stricter safety regulations were imposed on race organizers.
For the 1980 Grand Prix season, the Yamaha factory made the Yamaha USA team of Roberts and Carruthers the de facto factory racing team. The season got underway two months late due to cancellation of Austrian and Venezuelan rounds. Barry Sheene had been replaced by Randy Mamola as the top Suzuki rider as, Sheene had been dissatisfied with the Suzuki's efforts and had turned to a privateer Yamaha team. Roberts won the first three races as the Suzuki team appeared to be in disarray, but by the third race, the Suzukis of Mamola and Marco Lucchinelli were making things more difficult for Roberts. Roberts' Yamaha suffered a deflating front tire and a faulty rear shock absorber in the Netherlands forcing him to pull out of the race, but his main championship rivals also suffered setbacks with Cecotto, Ferrari and Hartog all missing races due to injuries and Sheene suffering mechanical breakdowns. Suzuki riders went on to win the last four races, but Roberts had built up a sufficient point lead to hold on and clinch his third consecutive 500 cc world championship.
In 1981, Yamaha introduced a new square-four cylinder bike, similar to Suzuki's RG500. Roberts raced to a second-place finish behind Marco Luchinelli at the non-championship Imola 200 race. Roberts' bike had a suspension failure in the Grand Prix season opener at Austria, but he rebounded to win the next two races in Germany and Italy. Roberts' title hopes suffered a setback at the Dutch TT at Assen when his Yamaha's front brake pads were installed incorrectly causing his front wheel to lock up on the starting line, ending his race before it had started. He came back to score a second place behind Lucchinelli in Belgium, but was once again struck by misfortune when a bad case of food poisoning forced him to miss the San Marino Grand Prix. He then narrowly lost the British Grand Prix to Jack Middelburg by three-tenths of a second before ending his season with a seventh place in Finland and a retirement in Sweden. Suzuki team riders Mamola and Lucchinelli battled to the final race of the season before the Italian claimed the championship with a total of five Grand Prix victories, with Mamola finishing in second and Roberts in third place.
Roberts switched to Dunlop tires for the 1982 season, as Goodyear pulled out of motorcycle racing. New competition had arrived as Honda entered their new two-stroke NS500 ridden by defending champion Lucchinelli, former 350 cc world champion, Takazumi Katayama and newcomer Freddie Spencer. Roberts won the season-opening round in Argentina on the old square-four Yamaha, but then switched to the new OW61 YZR500 V4 engined bike. He came in third at the Austrian Grand Prix then, sat out the French Grand Prix at Nogaro as he and the other top riders boycotted the race over unsafe track conditions. Roberts then won the Spanish Grand Prix at Jarama ahead of Sheene, and scored a second place behind Suzuki rider Franco Uncini in the Netherlands. In a portent of things to come, Roberts was leading the Belgian Grand Prix when his Dunlop tires lost their grip and he had to settle for fourth place as Spencer went on to win his first Grand Prix for Honda. Roberts then injured his knee and finger at the British Grand Prix and had to miss the Swedish round, but by then the world championship had been claimed by Uncini with a total of five victories while Roberts fell to fourth place. By the end of the 1982 season, Roberts had won sixteen 500 cc Grand Prix races, more than double that of any of his contemporaries.
Roberts' riding style in which he forced the motorcycle's rear wheel to break traction to steer around a corner, essentially riding on paved surfaces as if they were dirt tracks, changed the way Grand Prix motorcycles were ridden. From 1983 to 1999, every 500 cc world championship was won by a rider with a dirt track racing background. Roberts' cornering method of hanging off the motorcycles with his knee extended forced him to use duct tape as knee pads, and eventually led to the introduction of purpose-built knee pucks used by all motorcycle road racers today. His battles with the Grand Prix establishment eventually led to the adoption of stricter safety standards for Grand Prix race organizers. He was one of the first riders to challenge the FIM over the way they treated competitors and helped improve prize money as well as the professionalism of the sport. It was not until Roberts planned his rival race series in 1980 that the FIM was forced to change the way in which they dealt with motorcycle racers.
Roberts continued to ride in selected events in 1984. In March, he battled Spencer to win his second consecutive Daytona 200 and third win overall. In July, Roberts won the first leg of the Laguna Seca 200, then finished second to Randy Mamola in the second leg, as Mamola was declared the winner based on aggregate times. In September 1985, he appeared at the Springfield Mile Grand National dirt track race riding a Mert Lawwill-prepared Harley-Davidson XR750, but failed to make the final.
In July 1985, Roberts won the pole position at the prestigious Suzuka 8 Hours endurance race, held in Japan. Teamed with Tadahiko Taira, the duo were leading the race until the final hour, when mechanical problems dropped them back to seventeenth place. Roberts returned to compete in the 1986 Suzuka 8 Hours, this time teaming up with American Mike Baldwin. He qualified second behind Wayne Gardner, but failed to finish the race.
Throughout his career, Roberts has been a strong proponent of raising the image of motorcycle racing among the general public. During his riding career, he made a point of returning to the United States during the mid-season break in the Grand Prix calendar to race in the Laguna Seca 200 as a way to increase the profile of the event in order for it to gain Grand Prix status. The race eventually attained Grand Prix status in 1988 and in 1993, Roberts took on the role of promoter, providing financial backing for the 1993 United States Grand Prix. In the 1990s when Grand Prix racing faced diminishing numbers of competitors due to increasing costs, Roberts demanded that Yamaha provide engines to privateer teams in order to bolster the number of racers.
In 1997, Roberts stunned the racing world when he left Yamaha after more than 25 years to start his own motorcycle company. Roberts had grown weary of battling over the direction he felt the Yamaha team needed to pursue. Basing his new company in England to take advantage of the Formula 1 industry, Roberts built a three-cylinder, two-stroke engine with the engineering assistance of Tom Walkinshaw Racing. He decided to take advantage of rules allowing lighter weights for three-cylinder motorcycles after observing the agility and handling advantage of Spencer's Honda NS500 during the 1983 season. Unfortunately, by the time the motorcycle had been developed, tire Technology had improved to the point where any advantage over four-cylinder bikes had been negated. The motorcycle did manage to win a pole position with rider Jeremy McWilliams taking the top qualifying position at the 2002 Australian Grand Prix against the new breed of 990 cc four-stroke MotoGP motorcycles.
Roberts' son, Kenny Roberts Jr., won the 2000 500 cc World Championship, making them the only father and son duo to have won the title. Ironically, Roberts has stated that he considers himself a dirt tracker at heart and only took up road racing because it was necessary to do so if a rider was going to compete for the Grand National championship. He also said that he would have preferred to remain in the United States to compete in the Grand National championship if Yamaha or another manufacturer had been able to construct a dirt track racer capable of competing with Harley-Davidson.
With the introduction of the MotoGP class in 2002, Roberts' team developed a five-cylinder bike called the KR5. The team was originally well-funded by Proton of Malaysia, but by the middle of the 2004 season, it became apparent that the Roberts team was not able to field an engine capable of competing with the dominant Japanese factories. Roberts turned to the KTM factory to provide engines for the 2005 season, however after ten races KTM abruptly withdrew their support on the eve of the Czech Republic Grand Prix, forcing the team to miss several races. Honda stepped in to help Roberts' team for the 2006 season by providing five-cylinder engines, as Robert's son, Kenny Roberts Jr., rode the Team Roberts KR211V bike to a sixth place in the championship including two podium results. The 2007 season saw the introduction of a new MotoGP engine formula using 800 cc four-stroke engines. Roberts would once again secure engines from Honda for the Team Roberts KR212V race bike, but the results were not as hoped, and funding for the team faded. After the 2007 season, Roberts pulled out of MotoGP competition due to the lack of sponsorship.
