Stommelen joined the Porsche factory sports car programme in the mid-1960s, winning the 1967 Targa Florio with Porsche. In 1968 he co-drove a Porsche 908 to first overall at the 24 Hours of Daytona. The following year he qualified on pole for the 1969 24 Hours of Le Mans in a Porsche 917 and, during practice, became the first driver to exceed 350 km/h (217 mph) on the Mulsanne Straight. In 1970 he made his Formula One debut with the Brabham team, racing concurrently in sportscars throughout the decade.
Stommelen contested 63 World Championship Grands Prix across nine seasons, achieving one podium and 14 championship points. His Formula One career was marked more by circumstance than consistent machinery; he drove for Brabham, Alfa Romeo, and Hill during his years in the championship.
The most consequential moment of his F1 career came at the 1975 Spanish Grand Prix at Montjuich Park in Barcelona. The rear wing of his Hill GH1 failed during the race, sending the car airborne into the crowd. Five spectators were killed and Stommelen was seriously injured. The accident ended the Spanish Grand Prix's long association with the Montjuich circuit.
After recovering from his injuries, Stommelen largely stepped away from Formula One to focus on sports car racing, where he excelled.
Stommelen's greatest achievements came in endurance racing. He won the 24 Hours of Daytona four times — in 1968, 1978, 1980, and 1982 — a record of consistency spanning fifteen years of competition.
His most celebrated single event was a 300 km race at the Nürburgring in 1976, driving the Porsche 936 on its maiden race outing. The car ran in an unpainted black body without the usual air-intake, becoming known as the "black widow." Stommelen led from the start ahead of two factory Renault Alpines, then deliberately held back to let them past at the Nordkehre — where both Renaults, eager to retake the lead after two of 300 kilometres, rushed into standing water and crashed into the catch fences in tandem. When the throttle cable of the 936 stuck in the open position later in the race, Stommelen completed it by turning the ignition off at corners to brake and on again along the straights to accelerate, taking second place. The episode gave rise to the saying on the Nordschleife: "You can never brake later than Rolf Stommelen."
In 1978, Porsche entrusted Stommelen with the 935 "Moby Dick" in Martini livery, its 3.2-litre turbocharged engine producing 845 bhp. He reached 235 mph (365 km/h) on the Mulsanne Straight at Le Mans — faster than the prototypes of that era — but excessive fuel consumption required too many pit stops to contend for victory.
In 1979 he came within 23 minutes of winning Le Mans outright, co-driving with Dick Barbour and actor Paul Newman, only for a stuck wheel nut to derail a podium challenge. He won the German GT Championship Deutsche Rennsport Meisterschaft in 1977 in a Porsche 935 for the Gelo Racing Team, and continued racing in the early 1980s with Kremer, Lancia, and Porsche prototypes.
On 24 April 1983, during the Los Angeles Times Grand Prix at Riverside International Raceway, Stommelen was co-driving a Porsche 935 entered by John Fitzpatrick Racing alongside Derek Bell. Shortly after taking over from Bell, running in second place, the rear wing of the car failed at 190 mph (306 km/h). The car struck a concrete wall, somersaulted, and caught fire. Stommelen died of blunt force trauma and crush injuries. He was 39.