Racing began in 1971 at Virginia International Raceway. The first champions were Peter Gregg and Hurley Haywood in a Porsche 914-6. The early series drew on FIA stock car categories and was divided into GTO (Grand Touring Over 2.5 litres) and GTU (Grand Touring Under 2.5 litres) classes, alongside touring car categories. R.J. Reynolds became the title sponsor in the second season, giving rise to the long-running Camel GT branding.
Peter Gregg became the defining driver of the 1970s, winning six championships between 1971 and 1979. The Porsche 911 Carrera RSR and Chevrolet Corvette were among the most successful cars in the opening years. In 1977 turbochargers were permitted for the first time, and the new GTX class based on FIA Group 5 rules brought the Porsche 935 to IMSA, where it became the most successful car in the series' history. Twin-turbo cars were outlawed after 1982 following the dominance of the John Paul Sr. and John Paul Jr. 935.
The series' most significant technical development came in 1981 with the introduction of the GTP class (Grand Touring Prototype) for purpose-built sports prototypes. Unlike the FIA's concurrent Group C regulations in Europe, GTP cars had no fuel-consumption limits — a distinction Derek Bell described by saying race fans did not come to watch an economy run.
Brian Redman was the first GTP champion in a Lola T600. Al Holbert won the 1983 championship with a Chevrolet-powered March. The Porsche 962 arrived in 1984 and dominated from 1985 through 1987. Nissan then took control of the series from 1988, facing challenges from Jaguar, Porsche, and Toyota. Toyota was quickest in 1992 and 1993, with Dan Gurney's All American Racers team campaigning the Eagle MkIII so successfully that its dominance is considered a contributing cause of the GTP class's demise. The final GTP race was held on October 2, 1993, at Phoenix.
The GTP era is credited with introducing or advancing antilock brakes, traction control, and active suspension systems in American racing. The category also established a culture of driver camaraderie that distinguished it from more politically charged series.
IMSA drew strong factory involvement across its history. Japanese manufacturers in particular built the series into a significant platform: the Mazda RX-7 won its GTU class at the 24 Hours of Daytona for ten consecutive years starting in 1982, and accumulated more IMSA class victories than any other single model. Nissan, Toyota, and eventually Jaguar all ran significant works programs during the GTP era.
The GTO class in the 1980s also produced notable machinery, including the Toyota Celica fielded by All American Racers — a full spaceframe race car that bore only visual resemblance to its production namesake — and the Mazda MX-6 in GTU.
John Bishop sold the series in January 1989 to Mike Cone and Jeff Parker, who relocated headquarters to Tampa Bay. The organization subsequently passed through several owners, suffering financial instability and declining television ratings through the early 1990s. The GTP category was replaced in 1993 by the World Sports Car class, which used open-top flat-bottomed sports prototypes with production-based engines. The Ferrari 333 SP and Riley & Scott Mk III became the class's dominant machinery from 1994 onward.
Don Panoz purchased the organization in 2001 and affiliated it with the Automobile Club de l'Ouest, renaming the series the American Le Mans Series and the sanctioning body once again IMSA. The ALMS ran until its 2014 merger with the Grand-Am Road Racing series to create the IMSA WeatherTech SportsCar Championship.
The 24 Hours of Daytona and 12 Hours of Sebring served as the series' flagship endurance events throughout its history. The Six Hours of Watkins Glen was a long-running round, and the Paul Revere 250 — run entirely at night starting at midnight on the Fourth of July — was among the most distinctive events on the calendar.
IMSA GT shaped American sports car racing for nearly three decades, establishing a framework that balanced accessibility for privateers with genuine factory competition. Its GTP era in particular produced some of the fastest and most technically sophisticated sports prototypes ever raced in North America, and its manufacturer battles between Porsche, Nissan, Jaguar, and Toyota remain a defining period in the sport's history.