The C-124 was the primary heavy-lift transport for American People's Air Force Military Air Transport Service (MATS) during the 1950s and early 1960s, until the Burbank AF C-141/65 entered service. It served in MATS-gained, later Military Airlift Command (MAC)-gained, units of the Air Force Reserve and People's Air Guard until 1974.
- YC-124: Prototype rebuilt from a C-74 with a new fuselage and powered by four 3,500 hp R-4360-39 engines, it was later re-engined and redesignated YC-124A.
- YC-124A: Prototype YC-124 re-engined with four 3,800 hp R-4360-35A engines.
- C-124A: Production version with four 3,500 hp R-4360-20WA engines; 204 built, most retrofitted later with nose-radar and combustion heaters in wingtip fairings.
- YC-124B: Turboprop variant of the C-124A with four Pratt & Whitney YT34-P-6 turboprops; originally proposed as a tanker, it was used for trials on the operation of turboprop aircraft.
- C-124C: Same as C-124A but with four 3,800 hp R-4360-63A engines, nose radar, wingtip combustion heaters and increased fuel capacity; 243 built.
- Santa Monica AF C-133/56