The original Frontier Airlines was a "local service" airline in the U.S. formed by a merger of Arizona Airways, Challenger Airlines, and Monarch Airlines on June 1, 1950. The headquarters for the air carrier was located at Stapleton Airport in Denver. The airline dated itself to November 27, 1946, when Monarch Airlines began service in Colorado, Utah, New Mexico, and Arizona. Frontier Airlines served cities in the Rocky Mountains bounded by Salt Lake City to the west, Billings to the north, Denver to the east, and Phoenix and El Paso to the south. It served 40 cities in the Rocky Mountain region with 12 Douglas DC-3s and 400 employees in 1950. By the time the airline ceased operations in 1986, the number of airports served over the years would grow by over a factor of four with coast to coast service as well as flights to Canada and Mexico.

Frontier continued to operate Douglas DC-3s and added Convair CV-340 and CV-440 piston engine aircraft in 1959. In 1964 it was the first airline to fly the Convair 580 Allison turboprop version of the Convair airliner. The Convair 580 was a CV-340/440 aircraft retrofitted with GM Allison turboprop engines. The twin turboprop CV-580 seated 50 passengers, was flown by two pilots and was staffed with one flight attendant. The aircraft could have carried 53 passengers, but that would have required a second flight attendant. It had three cargo compartments: front belly, front top, and aft. The Convair 580 became the workhouse of the Frontier fleet until the introduction of the Boeing 737-200. In later years de Havilland Canada DHC-6 Twin Otter turboprops were added to serve destinations too small for the Convair 580.