Siffert was born in 1936 in Fribourg, Switzerland, to a poor family whose father ran a dairy. At 12, his father took him to Bern to watch the 1948 Swiss Grand Prix at the Bremgarten circuit, an experience that fixed his ambition to become a racing driver. He began his competitive career on two wheels, winning the Swiss 350 cc motorcycle championship in 1959, before switching to four-wheeled racing with a Formula Junior Stanguellini.
Siffert graduated to Formula One as a privateer in 1962 with a four-cylinder Lotus-Climax. He moved to Swiss team Scuderia Filipinetti before joining the respected private British outfit Rob Walker Racing Team in 1964. Early highlights in non-championship racing included victories at the 1964 and 1965 Mediterranean Grands Prix, both times beating Jim Clark by very narrow margins.
His first World Championship victory 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 in a race-long battle. The win is widely regarded as the last Formula One Grand Prix victory by a genuine privateer in the World Championship era. His second championship win followed in 1971 at the Austrian Grand Prix at the Österreichring, this time driving for the BRM factory team. Over ten seasons in Formula One, Siffert was a persistent, respected competitor whose privateer results frequently outpaced far better-resourced factory entries.
While Siffert's F1 profile grew steadily, his greatest successes came in endurance racing as one of Porsche's lead factory drivers in their campaign for the World Sportscar Championship. In 1968, paired with Hans Herrmann, he won both the 24 Hours of Daytona and the 12 Hours of Sebring in a Porsche 907 — among the first major outright victories for the Stuttgart manufacturer on the world stage.
Driving the fearsome Porsche 917, Siffert accumulated multiple major wins across Europe. He also participated in Porsche's CanAm development programme, racing a Porsche 917PA spyder in 1969 and finishing fourth in the championship. In 1970, paired with Brian Redman in a Porsche 908/3, he won the Targa Florio. That same year, concerned about losing Siffert to Ferrari, Porsche funded his seat at March Engineering in Formula One. The arrangement proved unsatisfying, leading him to join Pedro Rodriguez at BRM for 1971.
Among his endurance racing results: three victories at the 1000 km of Zeltweg (1968, 1969, 1970), two victories each at the 1000 km of Spa-Francorchamps (1969, 1970) and the 1000 km of Nürburgring (1968, 1969), plus wins at the 1000 km of Monza (1969) and 6 Hours of Watkins Glen (1969).
On 24 October 1971, Siffert was competing in the non-championship World Championship Victory Race at Brands Hatch — the very circuit where he had taken his famous 1968 Formula One victory. On the opening lap, a collision with Ronnie Peterson damaged the suspension of his BRM. The weakened suspension later broke completely. His car crashed and immediately caught fire. Three fire extinguishers failed to operate correctly, delaying rescuers for five minutes. Siffert was found to have suffered only a leg fracture in the initial crash; he died of smoke inhalation. The subsequent Royal Automobile Club investigation into the incident directly led to mandatory on-board BCF fire extinguishers and piped air systems for drivers, marking a significant advance in Formula One safety.
Siffert's funeral in Fribourg was attended by an estimated 50,000 people. A Gulf-Porsche 917 of Team John Wyer led the hearse through the streets of his home city.
Jo Siffert occupied an unusual position in motorsport: a privateer who consistently ran with and beat factory drivers, and a sports car specialist whose achievements with Porsche helped transform the manufacturer into a dominant force in endurance racing. His 1968 British Grand Prix victory stands as a landmark in Formula One history — the last win by a true privateer entrant. Safety changes triggered by his death at Brands Hatch saved lives in the years that followed. In 2007–08, A1 Team Switzerland carried a commemorative message marking the 40th anniversary of his Brands Hatch victory during the final round of the A1GP season at the same circuit.