Construction and early growth (1942–1967)

A shared public airport was proposed by the Port of Seattle and Port of Tacoma in the late 1920s, but deferred plans after the legality of public ports operating such facilities was rejected by the state attorney general in a published opinion. The state legislature authorized municipal corporations such as public ports to establish aviation facilities in 1941. The Port of Seattle accepted a $1 million grant (equivalent to $15.1 million in 2024 dollars) from the Civil Aeronautics Administration in March 1942 to construct a new airport to serve the Seattle area after the U.S. military took control of Boeing Field during World War II. A site on U.S. Route 99 near Bow Lake south of Seattle was chosen at the end of the month ahead of another candidate near Lake Sammamish that was considered too close to the Cascade Range. The City of Tacoma provided $100,000 towards the airport's construction costs as part of a deal for the Bow Lake site, centered around an existing private airfield plagued by heavy fog. Construction of the airport, which was named Seattle–Tacoma in recognition of Tacoma's contribution, began with a groundbreaking ceremony on January 2, 1943.

The project was originally estimated to cost $1.7 million (equivalent to $24 million in 2024 dollars), but the sandy soil conditions drove the final construction price to over $4.2 million (equivalent to $59.3 million in 2024 dollars). The airport's plateau was formed through 6.5 million cubic yards (5,000,000 m3) of excavated earth. A proposal to rename the airport for Boeing president Philip G. Johnson shortly after his death in September 1944 was rejected by Port of Seattle commissioners due to the objections of Tacoma. The first ceremonial landing at Seattle–Tacoma Airport was made on October 31, 1944, by a United Air Lines DC-3 carrying local elected officials and civic leaders. The first commercial flights launched in May 1945 with Northwest Airlines, but use was limited due to the U.S. Army Air Force's need for the airport to stage Boeing B-29 bombers for delivery. Various airlines had irregular flights to the airport, which used a Quonset hut with limited heating as a terminal until a permanent building was financed by a bond issue that voters approved in 1946.

The first scheduled commercial flights began on September 1, 1947, with Northwest Airlines and Western Airlines operating ten daily departures. They were joined by United, Alaska, Trans-Canada, Western, and Pan Am by 1951 as airlines departed from Boeing Field. The terminal at the renamed Seattle–Tacoma International Airport was formally dedicated by Governor Arthur Langlie on July 9, 1949, in front of a crowd of 30,000 spectators. The 71,000-square-foot (6,600 m2) building, designed by architect Herman A. Moldenhour, included a rooftop control tower and glass curtain walls in the concourses. The 907-acre (367 ha) airport originally had four runways at 45-degree angles, between 5,000 and 6,100 feet (1,500 and 1,900 m) long, for crosswind operations. The two perpendicular runways were arranged into an "X"-shape that intersected near the longest, north–south runway; an additional runway to the south ran east–west. The terminal building's "inverted V" shape was arranged to match the runway layout. The north–south runway (now Runway 16L/34R) was lengthened to 7,500 ft (2,300 m) in 1950 at a cost of $900,000, to 8,500 ft (2,600 m) in 1955, and to 10,200 ft (3,100 m) in 1958 to support commercial jetliners.

The enlargement of the airport's runways was precipitated by the major augmentation of commercial air traffic in the 1950s. Larger aircraft such as the Douglas DC-6, which introduced a pressurized cabin; the DC-7; Lockheed Constellations; and Boeing 377 Stratocruisers were some of the last turboprops to fly long-distance from Seattle. By 1957, Sea-Tac hosted 400 flights per week, with United as the largest carrier, offering 260 of those. The Boeing 367-80 or Dash-80, the first aircraft to demonstrate the potential of jet aviation, was accommodated by the 1955 runway extension when it first flew into Sea-Tac on September 27, 1956. The first regular jet service to Sea-Tac was provided by Pan Am on October 3, 1959, when a Boeing 707 departed for Honolulu. In 1961 Alaska introduced Convair 880 service on its lucrative Anchorage–Seattle flights after Northwest and Pan Am had introduced the type on the same route, which reduced flying time to three hours from the eight-hour, multi-stop journey on piston-engined planes. Braniff and United introduced an interchange service between Houston and Seattle on DC-6s, where the airline crews switched on the stopover in Denver; by 1965 the DC-6s had been replaced by Boeing 720s. United, meanwhile became the first carrier to serve Sea-Tac with Douglas DC-8s when it introduced them on the routes to Chicago and Idlewild (now JFK) Airport in New York. By 1962 Northwest offered flights from Sea-Tac to six destinations, including Portland, Honolulu, Anchorage, Spokane, and Chicago, while Pan Am served Fairbanks, Honolulu and offered transatlantic service to London.