Roberts first competed in road racing in the 250cc class at Willow Springs in 1990, winning five races in his debut season. By 1993, he made his World 500cc debut at Laguna Seca and raced full-time in the 250cc World Championship for 1994 and 1995 with the Marlboro Yamaha team. He moved up to 500cc World Championship racing with Yamaha in 1996, finishing 13th overall in his debut season, after which Yamaha declined to renew his contract.
Roberts joined his father's independent team in 1997, spending two seasons developing the Modenas two-stroke machine. The project was competitive in ambition but yielded modest results: he finished 16th in 1997 and 13th in 1998. Though difficult in terms of results, the development period gave Roberts a level of technical understanding and resilience that would prove valuable at Suzuki.
Suzuki signed Roberts for 1999 and the partnership immediately produced results. His debut race with Suzuki in Malaysia resulted in a victory, defeating the reigning champion Michael Doohan. He followed that with another win in Japan, again defeating Doohan. These results positioned Roberts as a genuine championship contender. After Doohan retired mid-season due to injury, Roberts' main rival became Doohan's teammate Alex Criville. Despite two further wins and four additional podiums, Roberts could not maintain consistency across the full season, and Criville ultimately took the title while Roberts finished a strong second.
In 2000, Roberts renewed his championship challenge. With Criville unable to defend his form, the primary competition came from Valentino Rossi, a rookie who had just won the 250cc title. Roberts responded to the challenge with four wins and five podiums across 16 races. He clinched the championship two rounds before the end of the season at the Rio Grand Prix after finishing sixth. His victory made him the first son of a former champion to win the title and gave Suzuki their first premier-class championship in six years, ending Honda's run of consecutive titles.
In 2001, Roberts and Suzuki faced the task of defending the title against a dominant Rossi. He managed only a single podium and finished the year eleventh. The 2002 season brought a transition to the new four-stroke MotoGP era with the 990cc Suzuki GSV-R. Roberts spent four difficult seasons between 2002 and 2005 developing the machine while being challenged internally by younger teammate John Hopkins, who finished ahead of Roberts in both 2003 and 2004. Roberts managed two podiums across that four-year period. Suzuki chose not to renew his contract at the end of 2005, opting for Chris Vermeulen.
Roberts returned to his father's independent team in 2006 using Honda RC211V V5 engines in a Roberts-designed frame designated the KR211V. He took two podium finishes that season — at Catalunya and Estoril — and finished sixth in the championship standings, his best result since the 2000 title. The 2007 season brought a new 800cc formula, and the team's KR212V chassis using Honda RC212V V4 power struggled as Honda prioritized the works Repsol team's development. After scoring only four points in the first part of 2007, Roberts stopped racing mid-season and did not return. The team did not participate in 2008.
Roberts remains one of only a handful of riders to win the premier class 500cc world title, doing so in a season that required overcoming a gifted rookie in Rossi. His championship secured Suzuki's return to the top of the standings and demonstrated the potential of competitive independent team development before the factory era of MotoGP fully took hold. His induction into the FIM MotoGP Hall of Fame in 2017 recognized a career defined by the 2000 title and the unique family achievement he shares with his father.