On January 12, 1852 Thurston County was formed out of Lewis County, at the time this was all part of Oregon Territory. For the bulk of the year it encompassed all of the Puget Sound region as well as the Olympic Peninsula. On December 22, 1852 the Counties of Pierce, King, Jefferson and Island were created from parts of Thurston giving the county its current borders. The county was named after the first Congressional delegate from Oregon Territory, Samuel Royal Thurston.

Washington Territory was created on March 2, 1853 comprising all the area north of the Columbia River that were once in Oregon Territory. This was the result of meetings held in 1851 and 1852. The first meeting, on August 29, 1851 was held at Cowlitz Landing. This meeting was inspired by a speech given by John B. Chapman at Olympia calling for local settlers to split from Oregon Territory. A document was written, called a “memorial to Congress”, and was published in The Oregonian and the Oregon Spectator. Copies made their way to Washington DC where Joseph Lane, Oregon Territory’s representative, introduced a bill on December 6, 1852. The bill was signed by President Millard Fillmore on March 2, 1853. There was also a meeting at Monticello, near Longview, which took place on November 25, 1852. Current historians contend that the “Monticello Convention” had no effect on the decision of Joseph Lane to introduce the legislation. The reasoning being that at the rate of travel for news, word of this meeting would have taken far too long to influence Joseph Lane, although he did have a copy by the time the legislation was being debated in February 1853.

The first Territorial Governor was Isaac Ingalls Stevens, appointed to the position by Franklin Pierce on March 17, 1853. It took until November 1853 for Stevens to reach Washington Territory where he took up his position in the Territorial Capital of Olympia.