Hendricks Army Airfield was a training base for B-17 pilots, operating from 1941 to 1946, located near Sebring, Florida, roughly 70 miles south of Orlando and 140 miles northwest of Miami. After the war the base was converted to Sebring Regional Airport, a facility that continued active civilian operations. Russian-American aeronautical engineer Alec Ulmann, seeking to establish an endurance race modeled on the 24 Hours of Le Mans, recognized the potential of the decommissioned runways and service roads for circuit use. The first race was held on New Year's Eve of 1950, attracting thirty cars from across North America. The Sam Collier 6 Hour Memorial was won by Frits Koster and Ralph Deshon in a Crosley Hot Shot that had been driven to the event under its own power by Victor Sharpe.
The first 12 Hours of Sebring was held on March 15, 1952, establishing what would become the race's permanent identity as an endurance classic. For most of its early history the circuit followed a 5.200-mile layout, winding across the active airport's runway intersections and perimeter service roads. The surface was a patchwork of World War II-era concrete slabs separated by large expansion joints, producing a brutally rough ride that was exceptionally hard on machinery. Mario Andretti, a three-time winner at the circuit, noted that one of the hardest challenges of the original layout was simply finding the track, since the circuit was poorly demarcated from the surrounding airfield with little more than white lines and cones. Drivers occasionally retired not from mechanical failure but from becoming lost on the runway sections, even in daylight.
In 1959 Sebring hosted what was officially the first Formula One World Championship round held in the United States, the United States Grand Prix. The rough, bumpy surface and open airfield character of the circuit proved a poor showcase for Formula One: attendance was low and operating costs were high, and the next United States Grand Prix was relocated to Riverside International Raceway in California.
A catastrophic 1966 running of the 12 Hours resulted in five fatalities, including four spectators killed when a privately entered Porsche lost control and entered a crowd near one of the on-site warehouses and hangars that flanked the Warehouse Straight. In response the track was widened in places for 1967 and lengthened by approximately 50 yards. The Webster Turn between the hairpin and the top of the circuit was removed and replaced with the faster Green Park Chicane, which allowed a flat-out run to the top section while routing cars away from the dangerous warehouse buildings.
The circuit continued to evolve as the relationship between the raceway and the active airport required ongoing compromise. A 1983 reconfiguration shortened the layout to allow simultaneous use of the track and an active runway. Further changes in 1987 permitted use of a second runway while racing was in progress. By 1991 additional airport expansion drove another reconfiguration that brought the layout close to its modern form. The hairpin was eliminated in 1997 due to insufficient runoff and replaced with what became known as the safety pin section.
The original and early-revision Sebring layouts shared characteristics that distinguished them from any other major circuit of the era. The concrete runway sections and their large seams sent cars airborne or sparked their undercarriages at speed. The flat Florida terrain offered no elevation change and minimal camber, providing no natural drainage and creating treacherous conditions in rain. The circuit's industrial surroundings, active aircraft, open exposure, and minimal permanent infrastructure gave the early races a frontier character that became part of the Sebring legend. The transition from that airfield rawness to a more conventionally managed road racing facility was gradual, shaped as much by airport expansion and safety legislation as by any deliberate track development philosophy.