History
Origin
The International Motor Sports Association was incorporated in Connecticut on 23 June 1969 by John Bishop — a former executive director of the Sports Car Club of America — and his wife Peggy, financed by NASCAR founder Bill France Sr., who held 75 per cent of the stock to Bishop's 25.12 The first IMSA-sanctioned event was a Formula Vee and Formula Ford meeting at Pocono in October 1969, but Bishop abandoned open-wheel plans within a year and built a six-race GT series for 1971 instead.2 R.J. Reynolds' Camel sponsorship arrived in 1972, the Sebring 12 Hours joined the schedule in 1973, and the Camel GT became the centre of professional sports car racing in North America — run, unusually, as a family business: Peggy Bishop managed registration, timing, and scoring throughout.23
Turning points
The GTP era and the long decline
Bishop introduced the GTP prototype category in 1981, and the decade it defined — Porsche 962s against works Jaguars, Nissans, and Toyotas — remains the benchmark American sports car era.2 Bishop, rethinking his priorities after heart surgery, sold the company to Mike Cone and Jeff Parker in January 1989; ownership churned through the 1990s as manufacturer budgets inflated and left, GTP collapsed after 1993, and the organisation was even renamed Professional SportsCar Racing in 1997.12
The split
In 1998 Don Panoz staged Petit Le Mans at Road Atlanta under a partnership with the Automobile Club de l'Ouest, then launched the American Le Mans Series for 1999 on the ACO rulebook — a direct pipeline between American racing and Le Mans.2 Jim France answered with Grand-Am Road Racing, which debuted at the 2000 Rolex 24 at Daytona and from 2003 ran its own deliberately cheap Daytona Prototype formula.2 For fourteen seasons North America carried two rival sports car championships with incompatible cars, split manufacturer money, and divided fields — a schism the sport could not afford indefinitely.4
The merger
France and Panoz announced the merger at Daytona in September 2012: Grand-Am absorbed the ALMS, the combined series debuted at the 2014 Rolex 24, and the IMSA name was retained as sanctioning body — the TUDOR United SportsCar Championship, renamed the WeatherTech SportsCar Championship in 2016.41 The decisive structural move came in 2023, when IMSA's revived GTP class adopted the LMDh hybrid-prototype ruleset developed jointly with the ACO, making cars eligible for both the WeatherTech Championship and the World Endurance Championship — Porsche, Cadillac, Acura, and BMW launched the class, and the Rolex 24 and Sebring returned to being global-calendar fixtures rather than domestic ones.56
Today's shape
Porsche Penske Motorsport swept the 2025 GTP drivers', teams', and manufacturers' titles for the second consecutive season — Matt Campbell and Mathieu Jaminet clinched the drivers' crown with third place at Petit Le Mans, Roger Penske's fifteenth sports car championship — while the No. 31 Cadillac of Jack Aitken, Earl Bamber, and Frederik Vesti won the race itself.7 At the 2026 Rolex 24, Felipe Nasr took a record-tying third consecutive overall win — joining Hélio Castroneves and Peter Gregg — sharing the No. 7 Porsche 963 with Julien Andlauer and Laurin Heinrich, 1.5 seconds clear of the No. 31 Cadillac after a race interrupted by a record six-and-a-half-hour fog caution.8 The 2026 championship fields 45 full-season entries across four classes — 11 GTP, 12 LMP2, 8 GTD Pro, 14 GTD — growing to 54 cars for the five Michelin Endurance Cup rounds; Acura, BMW, Cadillac, and Porsche have all introduced Evo updates of their GTP hybrids, BMW's programme moved to Team WRT, and Aston Martin's Valkyrie remains the only Le Mans Hypercar in the field.96
Footnotes
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Wikipedia, "IMSA," accessed 11 June 2026, https://en.wikipedia.org/wiki/IMSA. ↩ ↩2 ↩3
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IMSA Hall of Fame, "Timeline," accessed 11 June 2026, https://www.imsahalloffame.com/timeline/. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7
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IMSA Hall of Fame, "John Bishop and Peggy Bishop," accessed 11 June 2026, https://www.imsahalloffame.com/drivers/john-bishop/. ↩
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ESPN, "Grand-Am and ALMS merge into United SportsCar Racing," accessed 11 June 2026, https://www.espn.co.uk/racing/alms/story/_/id/9053893/grand-alms-merge-united-sportscar-racing. ↩ ↩2
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FIA WEC, "Major global marques headline 2026 FIA WEC entry list," accessed 11 June 2026, https://www.fiawec.com/en/news/major-global-marques-headline-2026-fia-wec-entry-list/8580. ↩
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IMSA, "IMSA GTP Field Nearly Complete for 2026 Entering December," accessed 11 June 2026, https://www.imsa.com/news/2025/12/03/imsa-gtp-field-nearly-complete-for-2026-entering-december/. ↩ ↩2
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IMSA, "Cadillac Wins Motul Petit Le Mans as Champions Crowned," accessed 11 June 2026, https://www.imsa.com/news/2025/10/11/cadillac-wins-motul-petit-le-mans-as-champions-crowned/; RACER, "Whelen Cadillac wins Petit Le Mans as Porsche crowned IMSA GTP champions," accessed 11 June 2026, https://racer.com/2025/10/11/Whelen-Cadillac-wins-Petit-Le-Mans-as-Porsche-crowned-IMSA-GTP-champions. ↩
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NASCAR, "Porsche Penske Motorsport collects third straight Rolex 24 win at Daytona," accessed 11 June 2026, https://www.nascar.com/news-media/2026/01/25/rolex-24-at-daytona-recap-porsche-penske-third-straight-win/; NBC Sports, "2026 Rolex 24 results: A Porsche Penske three-peat as Felipe Nasr closes another win," accessed 11 June 2026, https://www.nbcsports.com/motor-sports/news/2026-rolex-24-results-porsche-roger-penske-motorsport-three-peat-felipe-nasr-win-imsa-daytona-international-speedway. ↩
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IMSA, "Full Fields Continue For 2026 IMSA WeatherTech Championship, Michelin Endurance Cup," accessed 11 June 2026, https://www.imsa.com/news/2025/10/09/full-fields-continue-for-2026-imsa-weathertech-championship-michelin-endurance-cup/. ↩