The two seaports, which had been rivals for most of the 20th century but lost ground to nearby ports in British Columbia, proposed a merger of marine cargo operations in 2014.

A public development authority was created in 2015 and approved by the Federal Maritime Commission, resulting in the formation of The Northwest Seaport Alliance on August 4, 2015.

The two ports, located 32 miles (51 km) apart,[1] facilitated exports of the state's natural resources and imports from Asia, which intensified during World War I at the end of the decade.

[2] The rise of cargo containerization in the 1960s helped offset declining traffic to the Port of Seattle, leading to a $80 million modernization and expansion program begun in 1968.

[8] Both ports were supported by taxpayer subsidies by their respective counties and lost revenue as shipping and terminal operators used their rivalry to negotiate lower rates.

[15] The port commissions began talks of a merger in early 2014, including an agreement to share rates and other information under federal oversight and an "unusual" joint announcement for the transfer of a local shipping line to Tacoma.

[19] In March 2015, the plan was pushed back to August while working on specifics of the alliance, including a decision on which legal entity it would fall under.

[23] On August 4, 2015, at the Federal Way city hall, the two port commissions unanimously approved a formal agreement creating The Northwest Seaport Alliance.

[25] On February 29, 2016, the Northwest Seaport Alliance invited the CMA CGM Benjamin Franklin, the largest cargo ship to visit the United States, to dock at the Port of Seattle's Terminal 18.

[32] It is governed by the port commissions of Seattle and Tacoma, whose members are elected by the citizens of King and Pierce counties, respectively, to four-year terms.

[45] The Washington State Department of Transportation plans to enhance road connections between the two ports and Seattle–Tacoma International Airport through the Puget Sound Gateway Program.