Stewart was born in Milton, Dunbartonshire, a village fifteen miles west of Glasgow. His father ran an Austin and Jaguar car dealership, and his brother Jimmy competed as a racing driver with Ecurie Ecosse, including a start at the 1953 British Grand Prix. Stewart himself showed exceptional hand-eye coordination from an early age, winning the British, Irish, Welsh, and Scottish skeet shooting championships and twice winning the European "Coupe de Nations" championship. He narrowly missed selection for the British team at the 1960 Summer Olympics in trap shooting.
Stewart experienced severe difficulties at school due to undiagnosed dyslexia — he was not diagnosed until 1980, when his son Mark received the same diagnosis — and left formal education at sixteen to work as an apprentice mechanic in his father's garage. His driving talent emerged when a customer offered him track time at Oulton Park in 1961. By 1963 he was winning fourteen races in a season. Ken Tyrrell, then running a Formula Junior team, arranged a tryout at Goodwood in 1964 where Stewart, testing a Cooper T72-BMC, consistently bettered the times of Bruce McLaren, who had also been testing. Tyrrell offered him a place immediately.
Stewart drove in Formula Three for Tyrrell in 1964, winning the championship. He joined BRM alongside Graham Hill for the 1965 Formula One season, and before the season was over he had won his first World Championship race at Monza. He finished third in the Drivers' Championship in his debut season.
In 1966, while still with BRM, Stewart won the Monaco Grand Prix — the lone bright spot in a difficult season with unreliable H16 engines — and nearly won the Indianapolis 500 on his first attempt in John Mecom's Lola T90-Ford, leading by over a lap with eight laps remaining before a mechanical failure ended his race. That year also brought the pivotal accident that defined the rest of his career: at the Belgian Grand Prix at Spa-Francorchamps, he crashed at 165 mph in heavy rain and was trapped in a fuel-filled cockpit for an extended period without proper medical facilities or rescue tools. Fellow driver Graham Hill freed him, but it took an ambulance that became lost to reach hospital.
For 1968, Stewart moved to Ken Tyrrell's Matra International team. Driving the Matra MS10-Cosworth, he won at Zandvoort, in dramatic rain and fog at the Nürburgring by four minutes, and at Watkins Glen, but a retirement at the final race in Mexico cost him the title to Graham Hill. In 1969 with the Matra MS80, Stewart dominated comprehensively, winning at Kyalami, Montjuïc, Clermont-Ferrand, Silverstone, Zandvoort, and Monza. He led at least one lap of every World Championship Grand Prix that year — a feat no driver has since repeated — and became world champion. He was, until 2005, the only driver to win the title in a car built by a French constructor.
After a transitional 1970 season driving a March while Tyrrell built his own car, Stewart won the 1971 championship with the Tyrrell 003, claiming victories in Spain, Monaco, France, Britain, Germany, and Canada despite suffering mononucleosis during the season and crossing the Atlantic 186 times for media commitments. He was runner-up to Emerson Fittipaldi in 1972. Entering 1973, Stewart had already decided it would be his last season. He won four more Grands Prix, with his 27th and final victory at the Nürburgring setting a new all-time record. After his teammate François Cevert was killed in practice for the United States Grand Prix at Watkins Glen, Stewart retired one race earlier than planned, missing what would have been his 100th Grand Prix start.
Stewart's Spa accident in 1966 galvanised him into becoming the most prominent safety campaigner in Grand Prix history. He hired a private doctor to attend his races, taped a spanner to his steering shaft in case extraction equipment was unavailable again, and campaigned relentlessly for mandatory seat belts, full-face helmets, proper crash barriers, run-off areas, and medical facilities at circuits. He organised driver boycotts of Spa-Francorchamps in 1969, the Nürburgring in 1970, and Zandvoort in 1972 until safety improvements were made. Some drivers, journalists, and track owners resented the changes and the costs involved, but the reforms Stewart pushed through saved countless lives. He later said: "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 became a prominent television commentator for ABC's Wide World of Sports, covering Formula One, NASCAR, and the Indianapolis 500 for over a decade, and commented on athletics and equestrian events at the 1976 Olympics. His dyslexia meant he read from notes rather than an autocue throughout his broadcasting career. He also served for twenty-five years as a consultant to the Ford Motor Company.
In 1997, Stewart founded Stewart Grand Prix in partnership with his son Paul, entering the team in Formula One as a works Ford operation. The team's highlight came when Johnny Herbert won the 1999 European Grand Prix at the Nürburgring. Ford purchased the team in 2000, rebranding it as Jaguar Racing, which subsequently became Red Bull Racing in 2005.
Stewart received the OBE in 1971 and was knighted in 2001, both honours for services to motor racing. In 2018 he established the charity Race Against Dementia after his wife Helen was diagnosed with frontotemporal dementia. He was inducted into the International Motorsports Hall of Fame in 1990. Following the death of John Surtees in 2017, Stewart became the last surviving Formula One World Champion from the 1960s and is the oldest living Grand Prix winner.
Stewart held the record for most Formula One victories for fourteen years until Alain Prost surpassed him at the 1987 Portuguese Grand Prix, and the record for most victories by a British driver for nineteen years until Nigel Mansell broke it in 1992. He was the only British driver with three championships until Lewis Hamilton equalled him in 2015. Quantitative analyses have consistently placed Stewart among the top five Formula One drivers of all time. The Economist in 2020 ranked him fourth behind Fangio, Jim Clark, and Prost. Original F1metrics (2014) and updated F1metrics (2019) placed him second overall.