Stewart was born in Milton, Dunbartonshire. His family ran a successful Austin and Jaguar car dealership, and his brother Jimmy was a racing driver. Stewart experienced undiagnosed dyslexia throughout his schooling, leaving secondary education at 16 to work as a mechanic in the family garage; his dyslexia was not formally diagnosed until 1980. He became an accomplished clay pigeon shooter as a teenager, winning the British, Irish, Welsh, and Scottish skeet championships and competing for a place on the British Olympic team in 1960.
His path into racing began when a customer, Barry Filer, invited him to test cars at Oulton Park in 1961. By 1963, Stewart was winning prolifically in club racing. Ken Tyrrell, who ran the Cooper Formula Junior team, heard of the young Scot and offered him a test at Goodwood in 1964. Stewart outpaced Bruce McLaren in the test and earned a seat in Formula Three.
Stewart dominated British Formula 3 in 1964 without a single mechanical failure costing him a race he could have won. Despite receiving offers to jump directly to Formula One, he preferred to gain more experience under Tyrrell's guidance. He moved to F1 with BRM for 1965, finishing sixth on his championship debut in South Africa before winning his first championship race at Monza that year, ending the season third in the drivers' standings.
The BRM period produced mixed results. A serious accident at Spa-Francorchamps in 1966, where Stewart was trapped in a fuel-flooded cockpit for half an hour after crashing at speed, became the defining moment of his off-track career. Rescued only by fellow drivers Graham Hill and Bob Bondurant with no proper medical facilities available, Stewart became an outspoken advocate for circuit safety — pressing for crash barriers, medical services, and run-off areas at tracks across Europe.
Switching to Tyrrell's Matra International team for 1968, Stewart took on a more managerial role, negotiating with sponsors and engine suppliers. He narrowly missed the 1968 title after his car failed in Mexico City. In 1969, driving the Matra MS80-Cosworth, Stewart was dominant: winning at Kyalami, Zandvoort, Montjuïc, Clermont-Ferrand, Silverstone, and Monza, he claimed his first World Championship. He led at least one lap of every world championship grand prix that season, a record that stands unrepeated.
Stewart won his second championship in 1971 in the Tyrrell 003, taking victories at Spain, Monaco, France, Britain, Germany, and Canada. He claimed a third title in 1973, despite deciding before the season began that he would retire at year's end. His planned final race would have been his 100th grand prix, but after the fatal qualifying accident of teammate François Cevert at Watkins Glen, Stewart withdrew from the United States Grand Prix. His 27th and record-setting win had come at the Nürburgring earlier that year.
Stewart's crash at Spa in 1966 — where he lay in an upturned BRM in a ditch for over thirty minutes, fuel pooling around him, with no marshals or medical staff arriving promptly — galvanized him. He campaigned alongside Louis Stanley of BRM for mandatory seat belts, full-face helmets, armco barriers in front of pits, improved medical facilities, and the removal of fuel churns from pit lanes. He organized driver boycotts of Spa-Francorchamps in 1969, the Nürburgring in 1970, and Zandvoort in 1972 until safety upgrades were made. His efforts were not universally welcomed; some in the press believed his advocacy detracted from the sport's character.
Between 1971 and 1986, Stewart served as a Formula One color commentator for ABC in the United States, also covering NASCAR and Indy car events including the Indianapolis 500. He had been a play-by-play announcer for luge and equestrian events at the 1976 Olympics. His Scottish accent and rapid delivery were widely noted; he later revealed he relied on written notes rather than autocue because of his dyslexia. After leaving ABC in 1986 over conflicts arising from his Ford commercial work, he covered CART races briefly for NBC in 1987 and 1988.
Between 1997 and 1999, Stewart co-owned and served as principal of Stewart Grand Prix, the Ford works Formula One team he founded with his son Paul. The team's best season was 1999, when it finished fourth in the constructors' championship, claimed a pole position through Rubens Barrichello in France, and won the European Grand Prix at the Nürburgring with Johnny Herbert. Ford purchased the team outright after 1999; it later became Red Bull Racing.
Stewart was the only British driver with three world championships until Lewis Hamilton equalled the mark in 2015. Following the death of John Surtees in 2017, he became the last surviving Formula One World Champion from the 1960s. He is also the oldest living grand prix winner. His sustained campaign for circuit safety is widely credited with transforming conditions that had been routinely fatal into an environment where drivers could survive incidents that would previously have killed them.