Rymer's competitive career began in club racing, where he won the BMCRC 250cc and 350cc Production Championships as well as the KRC 250-350 Production Championships in 1985. He also claimed the Marlboro Clubmans final round at Silverstone, which served as a showcase for the fastest club racers in the United Kingdom. Turning semi-professional in 1987, he won British National races and fully committed to professional international competition from 1988.
In 1989, Rymer made history by becoming the first British rider to win a race in the Superbike World Championship, taking victory in New Zealand. He also finished runner-up in the British Formula TT Championship that year and won the prestigious Mallory Park Race of the Year, a victory he repeated in 1990. In 1990, Rymer also clinched the British Superbike Championship. He was a regular World Superbike competitor from 1988 to 1994, completing two full-time seasons and recording two race wins along with nine further podiums, with a best championship finish of sixth overall in 1991. Following his return to the British Superbike Championship, he recorded five race wins in 1996 with a Ducati and one further win in 1998 for Suzuki.
In a rare excursion to Grand Prix motorcycle racing, Rymer rode a privateer Harris-Yamaha at the 1992 British motorcycle Grand Prix and placed sixth. He also made a handful of 500cc Grand Prix starts for the Suzuki factory team in 1996.
Rymer achieved notable success in long-distance endurance racing. He won the FIM Endurance World Championship in 1992 and again in 1999, the latter title marking the end of his motorcycle career. He was a five-time winner of the Bol d'Or and claimed victories in both the 24 Hours of Le Mans and the Spa 24 Hour motorcycle race on two occasions each. At the Suzuka 8 Hours in 1994, Rymer and teammate Scott Russell finished second in what was recorded as the closest ever margin between first and second place in the event's history.
After retiring from motorcycle competition at age 32 following his second Endurance World Championship, Rymer pursued car and truck racing for a further three years. He campaigned Porsche GT cars and a MAN truck, winning seven races and earning multiple podium finishes across Britain and Europe. He also competed at the 12 Hours of Sebring in Florida.
Following the conclusion of his active racing career, Rymer remained involved in the sport through management of professional riders, commentary work for Eurosport, and track day instruction at circuits throughout the UK and Europe.