Mason County is a county located in the U.S. state of Washington. As of the 2020 census, the population was 65,726. The county seat and only incorporated city is Shelton. The county was formed out of Thurston County on March 13, 1854. Originally named Sawamish County, it took its present name in 1864 in honor of Charles H. Mason, the first Secretary of Washington Territory.

Shelton is a city in and the county seat of Mason County, Washington, United States. Shelton is the westernmost city on Puget Sound. The population was 10,371 at the 2020 census. Shelton has a council–manager form of government and was the last city in Washington to use a mayor–commission form of government.