The spectacular growth of air travel brought millions of dollars in commerce to the area around Sea-Tac International Airport. Hotels, parking lots, restaurants, catering companies, and related businesses generated tax revenues that went to King County, but was not necessarily reflected in services such as fire protection and law enforcement. In January 1988, local leaders filed a petition for incorporation signed by 850 voters, 60 percent more than needed to place the matter into consideration.
The City of Des Moines had annexation designs on some of the territory identified as part of the new city of SeaTac. In an agreement with Sea-Tac Citizens for Improved Services and Controlled Taxation, Des Moines removed 80 acres from 280 acres they planned to annex and the Committee dropped 200 acres from its plans. This set S 208th Street as a more natural boundary between the cities.
In November 1988, the King County Boundary Review Board approved the petition, but minus a portion of Pacific Highway S, which incorporation supporters had hoped to include in plans to clean up crime, and minus other neighborhoods which had applied to be annexed to Tukwila, Kent, or Des Moines.
The King County Council ruled that annexation and incorporation votes would be by mail and voters were given until March 14, 1989, to mail in or drop off their ballots. The ballots listed the new city as Sea-Tac. There was no cost savings between mail-in and in-person voting, but officials noted as much as a 20 percent better turnout by mail. The final tally was 2,870 to 2,710 in favor of incorporation. On the same day, Federal Way voters went for incorporation, but Woodinville voters turned the idea down.
SeaTac officially became a city on February 28, 1990, with a celebration involving a student poster contest, a three-and-a-half-foot ice sculpture, the Tyee High School Band, and the Seattle Christian School Choir.