The original Frontier Airlines dates to November 27, 1946, when Monarch Air Lines began service in the Four Corners states of Colorado, Utah, New Mexico, and Arizona. Frontier 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.

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In 1950, it flew to forty cities in the Rocky Mountain region with twelve Douglas DC-3s and 400 employees. Before ceasing operations in 1986, it flew to more than 170 airports at various times over the years, with service to both the U.S. east coast and west coast as well as to Canada and Mexico with an all-jet fleet.

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Frontier continued to operate Douglas DC-3s and added Convair CV-340s beginning in 1959; the company introduced a new logo on the new aircraft. On June 1, 1964 it was the first airline to fly the Convair 580, a CV-340/440 retrofitted with GM Allison turboprops. It had 50 seats, was flown by two pilots and carried one flight attendant. (The aircraft could have carried 53 passengers, but that would have required a second flight attendant.) The CV-580 was the workhouse of the Frontier fleet until the introduction of the Boeing 737-200s in the early 1970s. In later years de Havilland Canada DHC-6 Twin Otters and Beech 99s were added to serve cities too small for the Convair 580.

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In May 1958, Frontier flew to 40 airports; two years later it flew to 69. Half the additions had never seen an airline and several never would again, after Frontier pulled out. In May 1968, after merging with Central Airlines, Frontier flew to 100 airports, second among U.S. airlines (Pan Am was first with 122).

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In April 1958, Lewis Bergman "Bud" Maytag, Jr. (grandson of Frederick Louis Maytag I, founder of the Maytag Corporation), acquired controlling interest in Frontier. After all governmental approvals, he took control in January 1959 as chairman of the board and president. Three years later, Maytag sold his stock in March 1962 to the Goldfield Corp.;Lewis W. Dymond then became president of Frontier and, under his guidance, the airline entered the jet age with new Boeing 727-100s on September 30, 1966. The Boeing trijet was called the "Arrow-Jet" by the airline. The Boeing 727-200 became part of the fleet in February 1968.

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On October 1, 1967, Frontier purchased Central Airlines, headquartered in Fort Worth, Texas. The addition of Central added eleven Convair 600s and sixteen DC-3s to the fleet and many new cities. Convair 600s were Convair 240s that had been retrofitted with Rolls-Royce Darts; Frontier phased out the Convair 600s in 1969–70 and DC-3 flights ended in 1968. The Convair 580s lasted until May 31, 1982, when they were parked and eventually sold.

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Alvin Feldman became president in March 1971 and converted the jet fleet to Boeing 737-200s, eliminating the 727s. The 737-200 was Frontier's only jet type until McDonnell Douglas MD-80s were added beginning May 20, 1982.

On January 29, 1973, Frontier Airlines hired its first black pilot, Bob Ashby, the only Tuskegee Airman to become a commercial passenger airline pilot. It also hired the first female pilot for any modern day U.S. commercial airline the same day, Emily Howell Warner. Both were awarded their captain's wings several years later.

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According to the 1977 Frontier Airlines annual report, the airline was serving both Burbank (BUR) and Orange County (SNA) via an interchange flight agreement with Hughes Airwest with service between these southern California destinations and the Frontier hub in Denver (DEN) and had also begun serving Sacramento (SMF), its first directly served destination in California, that same year. Frontier would later introduce its own Orange County service as well as flights to a number of other destinations in California. This same annual report states the airline transported over 80 percent of its passenger traffic on board its growing fleet of Boeing 737-200 jets in 1977.

The final Frontier logo, a stylized "F", was created by Saul Bass and introduced April 30, 1978. By 1979, the airline had 5,100 employees and operated 35 Boeing 737-200 and 25 Convair 580 aircraft serving 94 cities in 26 states, Canada and Mexico.

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On February 1, 1980, Frontier president Al Feldman left to become the CEO of Continental Airlines. He was succeeded by Glen Ryland, and the airline started to decline. By 1982, employees began accepting lower wages and benefits in an effort to keep the business viable. Ryland resigned November 6, 1984, and was replaced by M.C. "Hank" Lund, the well-known vice president. Joe O'Gorman, from United Airlines, took over in May 1985, giving rise to speculation that United would buy Frontier.