Stewart was born in Milton, Dunbartonshire, into a family of Austin and later Jaguar car dealers. His father had been an amateur motorcycle racer, and his brother Jimmy drove for Ecurie Ecosse and competed in the 1953 British Grand Prix. Stewart attended Dumbarton Academy but was unable to continue past age 16 due to undiagnosed dyslexia, which was not formally identified until 1980 when he was 41. He became an apprentice mechanic in his father's garage.
Before discovering cars, Stewart excelled at clay pigeon shooting, winning the British, Irish, Welsh, and Scottish skeet championships, and competed for a place in the British trap shooting team for the 1960 Summer Olympics. Ken Tyrrell spotted him during trials at Goodwood in 1964, when Stewart was routinely beating Bruce McLaren's times in a Formula Three car. After Tyrrell offered him a spot, Stewart declined an immediate step to Formula One and instead spent 1964 winning the British Formula Three Championship.
Stewart signed with BRM alongside Graham Hill for 1965. His first World Championship race win came at Monza that year, and he finished third in the Drivers' Championship in his rookie season. The defining moment of the BRM era — and of his career — came at the 1966 Belgian Grand Prix at Spa-Francorchamps, where Stewart crashed at 265 km/h in heavy rain, his BRM pinned under a farm building with fuel pooling in the cockpit. With no marshals or tools available, he was eventually rescued by Graham Hill and Bob Bondurant who had also crashed nearby. The ambulance driver subsequently got lost en route to a Liège hospital. The experience turned Stewart into Formula One's most vocal safety campaigner.
He won the Monaco Grand Prix in 1966, one of his few highlights in a season marred by H16 BRM unreliability. At Indianapolis 500 that same year, Stewart came agonisingly close to a first-attempt victory in John Mecom's Lola, leading by over a lap with eight laps remaining when a broken scavenge pump ended his race. He still earned Rookie of the Year honours.
Stewart switched to Ken Tyrrell's Matra International team for 1968, driving the Matra MS10-Cosworth. Despite missing two races through injury, he won heavily at Zandvoort and the Nürburgring — the latter by four minutes — and at Watkins Glen, but a retirement at Mexico City handed the title to Graham Hill by a narrow margin.
In 1969, driving the Matra MS80, Stewart won the championship with a dominant campaign — winning at Kyalami, Montjuïc (by over two laps), Clermont-Ferrand (by over a minute), Silverstone (by over a lap), Zandvoort, and Monza. He led at least one lap of every World Championship Grand Prix that season, a feat no other driver has ever achieved. He remains the only driver to win the championship in a car built by a French constructor entered by a privateer team.
For 1970, a disagreement between Matra and Tyrrell over engines led Tyrrell to acquire a March 701 as a stopgap and then commission his own chassis, the Tyrrell 001. The season was transitional, with no championship title but growing promise.
Stewart claimed his second title in 1971 in the Tyrrell 003, winning Spain, Monaco, France, Britain, Germany, and Canada — even while managing mononucleosis during the season and crossing the Atlantic 186 times for media commitments. His third championship came in 1973, when he won at South Africa, Belgium, Monaco, and the Netherlands. His 27th and final victory, at the Nürburgring, set a new record for most Formula One wins at the time.
Entering 1973 Stewart had already privately decided to retire after the season finale at Watkins Glen, which was to have been his 100th Grand Prix start. When his teammate François Cevert was killed during qualifying at that race, Tyrrell withdrew both entries and Stewart retired one race earlier than planned. He never returned to competitive driving.
Stewart's 1966 Spa accident galvanised his lifelong campaign for circuit safety. Working alongside BRM's Louis Stanley, he pushed for mandatory seat belts, full-face helmets, proper barriers, run-off areas, fire crews, and track-side medical facilities. He organised driver boycotts of Spa in 1969, the Nürburgring in 1970, and Zandvoort in 1972 until conditions improved. He taped a spanner to his BRM's steering column as a personal emergency tool, and hired a private doctor to attend all his races. He later reflected: "I would have been a much more popular World Champion if I had always said what people wanted to hear. I might have been dead, but definitely more popular."
After retiring from driving, Stewart spent 15 years as a Formula One commentator for ABC Sports in the United States, covering Grands Prix, the Indianapolis 500, and the 1976 Winter Olympics among others. From 1997 to 1999 he co-founded and ran Stewart Grand Prix with his son Paul as Ford's works Formula One team, winning the 1999 European Grand Prix at the Nürburgring with Johnny Herbert before the team was sold to Ford and eventually became Jaguar Racing, then Red Bull Racing.
Stewart received knighthood in 2001 for services to motor racing. He won the Sports Illustrated Sportsman of the Year and BBC Sports Personality of the Year in 1973. In 2018, he founded the charity Race Against Dementia after his wife Helen was diagnosed with frontotemporal dementia. His record of 27 Formula One wins stood for 14 years until Alain Prost surpassed it in 1987; he was the only British driver with three championships until Lewis Hamilton equalled him in 2015.