Siffert was born in Fribourg, Switzerland, the son of a dairy owner. He grew up in modest circumstances and developed his passion for motorsport after attending the 1948 Swiss Grand Prix at Bremgarten with his father, aged twelve. He first made his name on two wheels, winning the Swiss 350 cc motorcycle championship in 1959, before switching to four-wheeled competition with a Formula Junior Stanguellini.
Siffert graduated to Formula One in 1962 as a privateer, initially with a four-cylinder Lotus-Climax. He subsequently drove for Swiss team Scuderia Filipinetti before joining Rob Walker's private Rob Walker Racing Team in 1964. During the mid-1960s he recorded early successes in non-championship events, winning the Mediterranean Grand Prix in both 1964 and 1965 — beating Jim Clark on each occasion by narrow margins.
His most celebrated Formula One achievement came at the 1968 British Grand Prix at Brands Hatch, where he drove Rob Walker's Lotus 49B to victory over Chris Amon's Ferrari after a race-long battle. That win is widely regarded as the last grand prix victory by a genuine privateer in the modern era. Siffert's second World Championship victory came in 1971 at the Austrian Grand Prix, this time as a works BRM driver at the Österreichring.
Across ten seasons in Formula One, Siffert was admired for his commitment and mechanical sensitivity, even when the equipment available to him was rarely equal to the front-running works machinery.
Siffert's partnership with Porsche gave him his greatest sustained success. In 1968, he and Hans Herrmann won both the 24 Hours of Daytona and the 12 Hours of Sebring in a Porsche 907, delivering the manufacturer its first major outright wins on high-speed circuits. Siffert went on to achieve multiple victories in the Porsche 917, one of the most powerful racing cars of its era, winning at circuits including Spa-Francorchamps, Zeltweg, and Monza across the 1968 to 1971 seasons. In 1969, he drove a Porsche 917PA spyder in CanAm, finishing fourth in the championship. In 1970, he and Brian Redman won the Targa Florio in a Porsche 908/3. Porsche valued Siffert so highly that the company sponsored his Formula One seat at March Engineering in 1970 to prevent him from joining rival Ferrari.
On 24 October 1971, Siffert was competing in the non-championship World Championship Victory Race at Brands Hatch. His BRM's suspension had been damaged in a first-lap collision with Ronnie Peterson and later broke completely, sending his car off the track. The BRM caught fire immediately. Three on-circuit fire extinguishers failed to function properly, delaying rescuers for five minutes. Siffert had survived the initial crash with only a leg fracture, but died of smoke inhalation. BRM initially concealed the mechanical cause of the accident; it was only revealed years later by a former mechanic.
The disaster had lasting consequences for racing safety. On-board fire extinguishers using BCF (bromochlorodifluoromethane) became mandatory, along with direct-air helmet feeds for drivers.
Siffert's funeral in Fribourg drew an attendance of 50,000 people. A Gulf-Porsche 917 of Team John Wyer led the hearse through the streets of the city. He remains one of the most beloved figures in Swiss motorsport history and a symbol of the privateer spirit in Formula One's transition era. In 2008, the A1 Team Switzerland car at Brands Hatch bore a tribute marking the 40th anniversary of his 1968 British Grand Prix victory at the same circuit.