Stewart grew up in Columbus, Indiana, beginning competitive karting in 1979 and winning a World Karting Association championship by 1987. He rose through USAC ranks to win the National Midget title in 1994 and 1995 and the Silver Crown championship in 1995, becoming the first driver to sweep USAC's three major divisions in a single year. He won the Indy Racing League championship in 1997, making him the only driver in history to win championships in both IndyCar and NASCAR. He transitioned to the NASCAR Winston Cup Series full-time in 1999 with Joe Gibbs Racing, setting a then-record for victories by a rookie with three wins and earning Rookie of the Year honours. He won Cup championships with Joe Gibbs Racing in 2002 and 2005.
In July 2008, Stewart entered into a co-ownership arrangement with Gene Haas, transforming Haas CNC Racing into Stewart–Haas Racing (SHR). Stewart took half ownership of the team and moved from Joe Gibbs Racing at the end of the 2008 season. The team entered competition in 2009 with Stewart driving the No. 14 Chevrolet — a number chosen as a tribute to Stewart's hero A. J. Foyt — and Ryan Newman driving the No. 39.
In his first season as owner-driver, Stewart won the non-points NASCAR Sprint All-Star Race and recorded his first points win as an owner at Pocono in June 2009, the first owner-driver to win a points race since Ricky Rudd in 1998.
The 2011 season was the high point of Stewart's tenure as an active owner-driver. Entering the Chase for the NASCAR Sprint Cup winless, he won five of the ten Chase races, including the season finale at Homestead-Miami Speedway. He finished level on points with Carl Edwards but claimed the championship on the tiebreaker — five wins to Edwards's one. The title made Stewart the first driver-owner to win the Cup championship in nearly two decades.
In 2014, Stewart was limited to a partial schedule after suffering broken leg bones in a sprint-car accident in August 2013 and was further affected by the fatal accident involving Kevin Ward Jr. in August 2014. Despite a difficult personal season for Stewart himself, the team's No. 4 Chevrolet driven by Kevin Harvick won the Cup championship, giving SHR its fourth Cup title and Stewart his first pure ownership title.
Under Stewart and Haas, SHR expanded to become a leading multi-car operation. The team signed Danica Patrick for 2013, making her the first woman to race full-time in the NASCAR Sprint Cup Series in decades. Kevin Harvick joined for 2014 and immediately delivered the owner's championship. Kurt Busch, Clint Bowyer, and other prominent drivers also competed under the SHR banner during the team's peak years.
Stewart drove his final full-time Cup season in 2016, after which Clint Bowyer took the No. 14. Stewart sold his stake in Stewart–Haas Racing, and the team subsequently announced in 2023 that it would cease Cup Series operations at the end of the 2024 season.
In November 2004, Stewart purchased Eldora Speedway, a half-mile dirt oval in New Weston, Ohio operating since 1954. He continued racing at the track in special events while using it to host prominent sprint car and midget car events, including an annual NASCAR Camping World Truck Series race on dirt. Eldora became a passion project for Stewart, reflecting his enduring connection to the grassroots open-wheel racing in which he first made his name.
Stewart was voted into the NASCAR Hall of Fame on May 22, 2019, and formally inducted on January 31, 2020, as part of the eleventh Hall of Fame class.