Shelton is a city in and the county seat of Mason County, Washington, United States. Shelton is the westernmost city on Puget Sound. The …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.