Thurston County was created out of Lewis County by the government of Oregon Territory on January 12, 1852. At that time, it covered all of the Puget Sound region and the Olympic Peninsula. On December 22 of the same year, Pierce, King, Island, and Jefferson counties were split off from Thurston County. It is named after Samuel R. Thurston, the Oregon Territory's first delegate to Congress.
According to the U.S. Census Bureau, the county has a total area of 774 square miles (2,000 km2), of which 722 square miles (1,870 km2) is land and 52 square miles (130 km2) (6.7%) is water.