Stewart was born in Milton, Dunbartonshire, Scotland, a village fifteen miles west of Glasgow. His family owned a successful business as Austin, and later Jaguar, car dealers. His father was an amateur motorcycle racer, and his brother Jimmy was a racing driver with a local reputation who drove for Ecurie Ecosse and competed in the 1953 British Grand Prix at Silverstone.
Stewart attended Hartfield primary school in Dumbarton and moved to Dumbarton Academy at age 12. He experienced learning difficulties due to undiagnosed dyslexia. Because the condition was not understood or widely known at the time, he was regularly berated and humiliated by teachers and peers. Stewart was unable to continue his secondary education past the age of 16 and began working in his father's garage as an apprentice mechanic. He was not diagnosed with dyslexia until 1980, when his oldest son Mark was diagnosed with the condition. Upon learning that dyslexia can be genetically passed on and seeing similar symptoms in his son that he had experienced as a child, Stewart requested to be tested and was diagnosed at 41 years old. He has said: "When you've got dyslexia and you find something you're good at, you put more into it than anyone else; you can't think the way of the clever folk, so you're always thinking out of the box."
Stewart's first car was a light green Austin A30, which he purchased for £375 shortly before his seventeenth birthday, saving the money from tips at the family garage. In 1961, Barry Filer, a customer of the family business, offered Stewart a chance to test his cars at Oulton Park. Filer provided a Marcos, in which Stewart scored four wins, and he also competed once in Filer's Aston DB4GT. In 1962, to help decide if he was ready to become a professional driver, Stewart tested a Jaguar E-type at Oulton Park, matching Roy Salvadori's times from the previous year. He won two races in the E-type, his first in England. David Murray of Ecurie Ecosse offered him a ride in the Tojeiro EE Mk2 and their Cooper T49, in which he won at Goodwood. In 1963, he earned fourteen wins, a second, and two thirds, with six retirements.
In 1964, Stewart signed with Ecurie Ecosse again. Ken Tyrrell, then running the Formula Junior team for the Cooper Car Company, heard of Stewart from Goodwood's track manager. Stewart came for a test at Goodwood, driving a new Formula Three T72-BMC that Bruce McLaren was testing. Stewart repeatedly bettered McLaren's times, leading Tyrrell to offer him a spot on the team. Stewart drove in Formula Three (F3) for Tyrrell, making a dominant debut in the wet at Snetterton on March 15, where he took a 25-second lead in two laps and won by 44 seconds. He declined an F1 ride with Cooper, preferring to gain experience under Tyrrell, and became F3 champion, failing to win only two races (one due to clutch failure, one to a spin). After practicing in a Ferrari at Le Mans, Stewart impressed Colin Chapman and Jim Clark during a trial in an F1 Lotus 33-Climax. He again refused an F1 ride, joining the Lotus Formula Two (F2) team instead, finishing second in his F2 debut at Circuit Clermont-Ferrand in a Lotus 32-Cosworth.
While Stewart signed with BRM alongside Graham Hill in 1965 for £4,000, his first F1 race was for Lotus, standing in for an injured Jim Clark at the non-championship Rand Grand Prix in December 1964. He qualified on pole, won the second heat, and claimed fastest lap. On his World Championship F1 debut in South Africa, he finished sixth. His first major competition victory came in the BRDC International Trophy in the late spring, and he won his first World Championship race at Monza, battling wheel-to-wheel with teammate Hill. Stewart finished his rookie season with a win, three seconds, a third, a fifth, and a sixth, placing third in the World Drivers' Championship. He also drove the Rover Company's turbine car at the 24 Hours of Le Mans alongside Graham Hill.
At the start of the 1966 season, Stewart won the Tasman Series from his BRM teammate Graham Hill. In F1, 1966 was a poor year due to unreliable 3-litre H16 BRMs, though Stewart won the Monaco Grand Prix in a 2-litre engined car. He almost won the Indianapolis 500 on his first attempt, leading by over a lap with eight laps to go in John Mecom's Lola T90-Ford, but a broken scavenge pump denied him the win. Despite this, his performance earned him Rookie of the Year honors. BRM's fortunes did not improve in 1967, and in F1, Stewart's highest finish was second at Spa, where he had to drive one-handed to hold the car in gear.
For 1968, Stewart switched to Tyrrell's Matra International team, driving a Matra MS10-Cosworth. He took a prominent role in team management, negotiating with sponsor François Guiter and leading presentations to Ford and Goodyear. After a promising start in South Africa, he missed Jarama and Monaco due to an F2 injury. His first win of the season was in heavy rain at Zandvoort, followed by another win in rain and fog at the Nürburgring, where he won by four minutes. He also won at Watkins Glen but lost the drivers' title to Hill due to a car failure in Mexico City.
In 1969, driving the Matra MS80-Cosworth, Stewart dominated several races, winning by over two laps at Montjuïc, a minute ahead at Clermont-Ferrand, and more than a lap at Silverstone. With additional wins at Kyalami, Zandvoort, and Monza, Stewart became world champion. He was the only driver to have won the championship in a car built by a French constructor and remains the only driver to win the world championship in a car built in France and entered by a privateer team. That year, Stewart also led at least one lap of every World Championship Grand Prix, a feat he remains the only driver to achieve.
For 1970, Matra insisted on using their V12 engines, while Tyrrell and Stewart preferred to continue with the Cosworth. Tyrrell decided to build his own car, and in the interim, Stewart drove a March 701-Cosworth to wins at the Daily Mail Race of Champions and Jarama. The new Tyrrell 001-Cosworth appeared in August, showing promise despite initial problems. Stewart continued to race sporadically in F2, winning at Crystal Palace. A projected Le Mans appearance with Steve McQueen in a Porsche 917K did not occur due to McQueen's inability to get insurance. He also had a one-off race in Can-Am in the Chaparral 2J, qualifying third before brake failure ended his race.
Stewart won the F1 World Championship in 1971 using the Tyrrell 003-Cosworth, with victories in Spain, Monaco, France, Britain, Germany, and Canada. He also completed a full season in Can-Am, driving a Carl Haas-sponsored Lola T260-Chevrolet, winning two races and finishing third in the championship. The stress of racing year-round and across continents led to medical problems. He won the 1971 world championship despite having mononucleosis and crossing the Atlantic Ocean 186 times for media commitments in the United States. During the 1972 season, he missed the Belgian Grand Prix due to gastritis. He won the Argentine, French, U.S., and Canadian Grands Prix, finishing second to Emerson Fittipaldi in the drivers' standings. Stewart also competed in a Ford Capri RS2600 in the European Touring Car Championship. In 1972, Stewart received the OBE.
Stewart’s advocacy for safety was sparked by a severe crash at Spa-Francorchamps in 1966. While driving at 165 mph (266 km/h) in heavy rain, he ran off the track and crashed into a telephone pole and a shed before coming to rest in a farmer's outbuilding. His steering column pinned his leg, and ruptured fuel tanks emptied their contents into the cockpit. There were no track crews to extricate him, nor were proper tools available. Stewart was rescued by fellow drivers Graham Hill and Bob Bondurant, who had also crashed nearby. There were no doctors or medical facilities at the track, and Stewart was placed in the bed of a pickup truck, remaining there until an ambulance arrived. He was first taken to the track's first aid center, where he waited on a stretcher on a floor strewn with cigarette ends and other rubbish. Another ambulance crew picked him up, but the driver got lost en route to a hospital in Liège. Ultimately, a private jet flew Stewart back to the UK for treatment.
After his crash at Spa, Stewart became an outspoken advocate for auto racing safety. He campaigned with Louis Stanley (BRM team director) for improved emergency services and better safety barriers around race tracks. He said: "We were racing at circuits where there were no crash barriers in front of the pits, and fuel was lying about in churns in the pit lane. A car could easily crash into the pits at any time. It was ridiculous." As a stop-gap measure, Stewart hired a private doctor to be at all his races and taped a spanner to the steering shaft of his BRM in case it would be needed again. Stewart pressed for mandatory seat belt usage and full-face helmets for drivers. He also pressed track owners to modernize their tracks, including organizing driver boycotts of races at Spa-Francorchamps in 1969, the Nürburgring in 1970 (joined by his close friend Jochen Rindt), and Zandvoort in 1972 until barriers, run-off areas, fire crews, and medical facilities were improved. Stewart later said: "If I have any legacy to leave the sport I hope it will be seen to be an area of safety because when I arrived in Grand Prix racing so-called precautions and safety measures were diabolical."
Stewart had decided to retire before the 1973 season. He revealed in 2023 on the "Beyond the Grid" podcast that he was getting depressed by the pace of his life and the limited time at home with his wife Helen and two young sons. He explained that to earn proper money in his era, he had to do many races beyond Formula 1, including Can-Am, Indianapolis, Touring Cars, and GT Cars, on a global basis. He noted that many of his friends were dying, with Helen counting 57 friends who had died that they had holidayed, traveled, and raced with. He felt it all got on top of him, especially after having mononucleosis in 1971 and gastritis that burst, leading him to question the extent of his racing. He had, by then, made good money.
He nevertheless won at South Africa, Belgium, Monaco, and the Netherlands in 1973. His last and then record-setting 27th victory came at the Nürburgring, securing a 1–2 finish for Tyrrell. He recalled: "Nothing gave me more satisfaction than to win at the Nürburgring and yet I was always afraid. When I left home for the German Grand Prix I always used to pause at the end of the driveway and take a long look back. I was never sure I'd come home again." Stewart had already won the Drivers' Championship at the Italian Grand Prix two races previously, a race where he drove from 20th to finish 4th after a flat tire. After the fatal crash of his teammate François Cevert in practice for the 1973 United States Grand Prix at Watkins Glen, Stewart retired one race earlier than intended, missing what would have been his 100th Grand Prix.
Stewart held the record for most wins by an F1 driver (27) for 14 years until Alain Prost won the 1987 Portuguese Grand Prix. He held the record for most wins by a British F1 driver for 19 years until Nigel Mansell won the 1992 British Grand Prix. In 2015, Lewis Hamilton equaled Stewart's record as the only British driver to win three championships. When John Surtees died in March 2017, Stewart became the last surviving F1 World Champion from the 1960s. He also became the oldest living F1 winner upon the death in 2022 of six-time race winner Tony Brooks.
Outside of Formula One, Stewart narrowly missed a win at his first attempt at the Indianapolis 500 in 1966. He competed in the Can-Am series in 1970 and 1971, finishing third in the 1971 championship. He also competed in sports car racing, including the 24 Hours of Le Mans, and the European Touring Car Championship. Stewart appeared in the 1966 John Frankenheimer movie Grand Prix, performing all the driving scenes for actor Brian Bedford, who did not know how to drive. Between 1997 and 1999, he served as team principal of the Stewart Grand Prix F1 racing team in partnership with his son, Paul.
Stewart received Sports Illustrated magazine's 1973 "Sportsman of the Year" award, the only auto racer to have won the title. In the same year, he also won BBC Television's "Sports Personality of the Year" award and was named ABC's Wide World of Sports Athlete of the Year, which he shared with American pro football player O. J. Simpson. In 1990, he was inducted into the International Motorsports Hall of Fame. In 1996, he was awarded an honorary doctorate by Heriot-Watt University in Edinburgh, and in 1998, he received an honorary doctorate from Cranfield University, where he later chaired the steering committee for the MSc Motorsport Engineering and Management.
Stewart was the subject of the television program This Is Your Life in January 1970. In the 1971 Birthday Honours, Stewart was created an Officer of the Order of the British Empire (OBE), and in 2001, he received a knighthood, both for services to motor racing. In 2002, he became a founding patron of the Scottish Sports Hall of Fame and an inaugural inductee. In 2003, the World Forum on the Future of Sport Shooting Activities presented Stewart the Sport Shooting Ambassador Award. On November 27, 2008, Stewart was awarded an honorary Doctor of Science (D.Sc.) degree from the University of St Andrews. On June 26, 2009, Stewart was awarded the Freedom of West Dunbartonshire in his hometown of Dumbarton. In 2010, Stewart was named a founding member of Motor Sport magazine's Hall of Fame.
In 2018, Stewart set up the charity Race Against Dementia, following his wife Helen's diagnosis with frontotemporal dementia. By that time, Helen had limited short-term memory and impaired mobility, requiring round-the-clock care. Stewart believes that applying Formula 1's technology and "out-of-the-box" thinking could bring about earlier solutions for society coping with dementia. By 2025, Helen's "horrendous" behavior and language changes had worsened, and she did not recognize Jackie when sitting beside him. Stewart's charity Race Against Dementia was funding the development of a blood test hoped to detect non-genetic frontotemporal dementia 10 to 20 years before it is otherwise diagnosed.