Early years

The airfield was founded in the late 1920s by Dr. Clifford "Kip" Strange, who needed space for his JN-4 "Jenny" Biplane. Known as Stroudwater Airport, the airport received its first commercial service on August 1, 1931, when Boston-Maine Airways began a flight from Portland to Boston. In 1937 the city of Portland purchased the airfield for $68,471 and changed its name to Portland-Westbrook Municipal Airport; this is the origin of its airport code, PWM. "Westbrook" referred to the location of the last directional light before the airport in the nearby city of Westbrook.

In January 1934, a statewide airport survey was conducted by Captain Harry M. Jones, of the Maine Emergency Relief Administration (MERA), a state division of the Federal New Deal public works programs launched in November 1933. MERA expended $816,376 across the state on labor in airport construction in the period April 1934 to July 1935. Two runways were constructed at Portland Municipal Airport by MERA, one north–south 2,400'x 100' gravel runway and one east–west 1,500'x 100' gravel runway. In the summer of 1935, the MERA aviation program had made possible the extension of the Boston-Portland-Augusta-Waterville-Bangor mail service to Bar Harbor, where an airport, Hancock County–Bar Harbor Airport, had been constructed by the MERA. The Portland town report of 1938 reported that the building of the runways and grading of the field were by WPA labor, and the city furnished part of the material. In 1940, the Works Progress Administration (WPA) built Portland's first real terminal, a brick structure that is now the general-aviation terminal.

According to Portland Town Reports, the WPA conducted two projects sponsored by the city:

"Improve municipal airport", official project number: 165‐1‐11‐111, total project cost: $669,295

"Complete construction of airport", official project number: 65‐1‐11‐2213, total project cost: $93,335

The present airport started to take shape in the 1950s. The March 1951 chart shows runway 1 4260 ft long, runway 10 2900 ft, and runway 15 4010 ft. Runway 11/29 was built in 1957 and lengthened to 6,800 feet (2,073 m) in 1966. The current terminal opened in 1968, when jet flights began.

1960s – 1970s

Northeast Airlines long had a monopoly on commercial air travel in Portland, dating to its time as Boston-Maine Airways. Another airline emerged in 1962, when Atlantic Airways began service to Boston's Logan International Airport. This competition was short lived; there is no other information about the airline other than one timetable.

Jet flights began in 1968 and, for the first time, Portland got a non-stop beyond Boston when Northeast DC-9s flew to LaGuardia Airport in New York City. Northeast would be alone at the airport until 1970, when Aroostook Airways began flights between Presque Isle and Portland, with stops in Augusta and Bangor. This airline too faded into obscurity, lasting until 1972.

That year, regional Air New England began service in Portland, competing with Northeast Airlines intrastate and between Portland and Boston.

In 1972, Northeast Airlines was bought by Delta Air Lines, which retained its routes to Bangor, Boston, and New York. By 1979, Delta had added Burlington, Vermont.

1980s – 1990s

In 1981, Air New England, after serving Portland for eleven years, ceased operations and pulled out of the Jetport. This departure was followed a year later by the arrival of Air Vermont, a regional carrier that flew between Portland and Burlington until expiring about 1983 or 1984.

In 1980, the passenger terminal expanded to the east with the addition of two baggage carousels. The building also expanded to the west by adding three second-level jetways and a holding room.

In 1982, PWM got its first non-stop flight beyond New York when Delta tried flights on Boeing 727 aircraft to Cincinnati for a year or so.

The Portland City Council renamed the Jetport for longtime senator Edmund Muskie in February 1982, but reversed its decision three weeks later after a public outcry and a request from Muskie to restore the original name.

People Express Airlines arrived in 1983, the first jet competitor to Northeast/Delta at PWM. The airline, the first low-cost carrier at the Jetport, was known for rock-bottom prices. The airline flew between Portland and Newark, still operated today by United Airlines, who merged with Continental Airlines, which had bought People Express in 1987.

In June 1983 United Airlines arrived in Portland, planning to be the only airline to serve 50 states. It originally flew the Burlington route that had been left behind by Air Vermont and later flew non-stop to Chicago.

That same year, regional Ransome Airlines, doing business as Delta Connection, began a route between Portland and Boston. This ended in 1986 when Ransome was bought by Pan Am and renamed Pan Am Express.

1986 also brought USAir (renamed US Airways, merged with American Airlines), which began flights to Philadelphia and Pittsburgh. Low-cost carrier Presidential Airways also began service from the Jetport in 1986, flying a route from Portland to Washington's Dulles International Airport.