Racing began in 1971 with a six-race calendar originally aimed at FIA GT and touring categories, divided into four classes: GTO (over 2.5-litre GT cars), GTU (under 2.5-litre GT cars), TO, and TU (equivalent touring classes). The first race at Virginia International Raceway was judged an unexpected success. The opening champions were Peter Gregg and Hurley Haywood sharing a Porsche 914-6 GTU.
R. J. Reynolds Tobacco's Camel brand came aboard as title sponsor from the second season, and the series became widely known as the Camel GT Challenge Series. Camel's branding requirements extended to the cars themselves: the corporate logo had to appear visibly on both sides of each racecar, and drivers were required to wear a Joe Camel patch on their Nomex suits.
Peter Gregg was the dominant driver of the 1970s, winning championships in 1971, 1973, 1974, 1975, 1978, and 1979. The Porsche 911 Carrera RSR and the Chevrolet Corvette were among the most successful cars of the early seasons. Starting fields of 30 or more competitors were routine; a signature event was the Paul Revere 250, run entirely at night starting at midnight on the Fourth of July.
In 1975 a new All American Grand Touring (AAGT) category was introduced to counter Porsche's dominance in GTO. Turbochargers were prohibited until mid-1977, when Porsche's protests led IMSA to permit them after officials examined Al Holbert's AAGT-winning Chevrolet Monza. The consequent GTX class (Grand Touring Experimental), based on FIA Group 5, brought overwhelming dominance by the Porsche 935, which became the most successful car in the series' history. Twin turbos were banned after the 1982 season following domination by John Paul Sr. and John Paul Jr.
In 1981, following a decision not to adopt FIA Group C regulations, Bishop introduced the GTP (Grand Touring Prototype) class for purpose-built prototypes. Brian Redman took the inaugural GTP title in a Chevrolet-powered Lola T600. Al Holbert won in 1983 and the Porsche 962 dominated from 1985 to 1987. Nissan then led the series from 1988, facing challenges from Jaguar, Porsche, and Toyota. Dan Gurney's All American Racers team campaigned the Eagle MkIII with such dominance in 1992 and 1993 that the car has been credited with contributing to the collapse of the GTP class. A secondary Camel Lights category for smaller non-turbocharged prototypes ran from 1985.
The GTP era generated lasting technical innovations introduced to American motorsport: antilock brakes, traction control, and active suspension all appeared in the class. The Red Lobster team of Dave Cowart and Kemper Miller is credited with pioneering modern race team hospitality practices that were subsequently adopted across the sport.
Bishop sold the organisation in January 1989 to Mike Cone and Jeff Parker, who relocated IMSA headquarters to Tampa Bay. After further ownership changes through Charles Slater and then Roberto Müller and Andy Evans, the series was renamed Professional Sports Car Racing in the mid-1990s. With rising costs, factory team withdrawals by Nissan and Mazda following the Japanese economic downturn, and declining television ratings, the GTP class held its last race at Phoenix on 2 October 1993.
A replacement World Sports Car (WSC) category for open-top flat-bottomed prototypes debuted in 1993. The Ferrari 333 SP became the headline car of the WSC era from its debut at Road Atlanta, where it won immediately. Riley & Scott's Mk III, powered by Oldsmobile and later Ford, emerged as Ferrari's main rival. The GT categories continued in parallel, undergoing several restructuring and renaming cycles throughout the 1990s.
In 1999, under the ownership of Don Panoz, the organisation affiliated with the Automobile Club de l'Ouest and adopted Le Mans-style regulations, relaunching as the American Le Mans Series. For legal continuity reasons, both the ALMS (1999–2013) and the current IMSA SportsCar Championship are regarded as successors to the original IMSA GT Championship, which ran under various names across nearly three decades of North American sportscar racing.
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