McWilliams entered Grand Prix racing in 1993, competing across the 250cc and 500cc classes through to 2001. His most significant results in the 500cc class came in 2000, when he took two podiums: one at the Italian Grand Prix and one at the British Grand Prix. In 2001, riding an Aprilia in the 250cc class, McWilliams won the Dutch round at Assen.
McWilliams joined Kenny Roberts' Team KR for the inaugural MotoGP season in 2002, riding the Proton KR three-cylinder two-stroke machine. At the Australian Grand Prix at Phillip Island, McWilliams took pole position — a remarkable achievement given that the Proton KR was a two-stroke competing against the four-stroke machines that had been designed specifically to replace two-strokes under the new technical regulations. The result stood as evidence of both McWilliams' qualifying ability and the Phillip Island circuit's characteristics favouring top-speed efficiency over outright power delivery. His best race result across the 2002 and 2003 seasons was sixth place at the 2003 French Grand Prix.
In 2004, McWilliams rode the Aprilia RS Cube in MotoGP alongside Shane Byrne. The Aprilia RS Cube was widely regarded as one of the most difficult machines to ride in the field, with a vicious power delivery that caused frequent crashes. McWilliams sustained broken ribs and a dislocated collarbone among other injuries during the season. His best finish was twelfth place, achieved three times.
For 2005, McWilliams was initially contracted to return to Team KR with KTM engine support, but Dorna intervened and gave the ride to Shane Byrne. McWilliams spent most of 2005 racing in the British Superbike Championship on a Honda, finishing outside the top ten partly due to a shoulder injury.
Later in 2005, McWilliams received a last-minute call to replace the absent Shane Byrne at the Czech Grand Prix at Brno, where Team KR found itself without a rider after KTM withdrew engine support with less than 24 hours' notice. McWilliams immediately posted lap times up to 1.5 seconds faster than the Proton V5 had recorded the previous year, but retired from the race due to a throttle sensor failure.
In 2007, McWilliams was contracted to race in MotoGP for the newly formed Ilmor team. He missed the season-opening Qatar round following a testing crash at the Losail International Circuit, and the Ilmor team suspended its racing operations after that round without returning.
McWilliams remained active in road racing events beyond the FIM age limit that made him ineligible for mainstream Grand Prix competition after 2014. In 2012 he finished second in the Super-Twin event at the North West 200. In 2014, aged 50, he made a one-off wildcard appearance in the Moto2 class at the British Grand Prix at Silverstone aboard a Brough Superior entry, finishing 29th.
In 2022 McWilliams joined the MotoAmerica King of the Bagger Series with the Indian Motorcycle factory team alongside 2021 class champion Tyler O'Hara. He took his first win in the series during the Daytona 200 weekend in 2022. He continued with the Indian factory team in 2023 in both the King of the Bagger and Super Hooligan classes. Aged 62, he won the Supertwin and Sportbike race at the 2026 North West 200, his fourth victory at that event.
McWilliams also holds a minor film credit, having appeared in a supporting role in Jonathan Glazer's 2013 film Under the Skin, starring Scarlett Johansson. Beyond racing, he has been involved in development work for motorcycle and tyre manufacturers including KTM, and runs race coaching schools under the McWilliams Masterclass banner.
McWilliams' career is defined as much by the circumstances he navigated as by raw results. His 2002 Australian Grand Prix pole position remains one of the more striking qualifying performances in early MotoGP history — a rider and a two-stroke machine threading through a transitional era to claim the fastest lap when the regulations had been written expressly to end the two-stroke's dominance. His longevity across different eras and disciplines, from 250cc and 500cc Grand Prix racing through MotoGP on multiple difficult machines to American bagger racing in his sixties, marks him as one of the more durable competitors in the sport's recent history.