Mason County was formed from Thurston County on March 13, 1854. It was originally named Sawamish County. The name was changed in 1864 in honor of Charles H. Mason, the first Secretary of Washington Territory. Shelton is the only incorporated city in the county. Other "Census designated places" in the county include Allyn-Grapeview, Belfair, Hoodsport, Kamilche, Skokomish, and Union. Unincorporated communities in the county include: Eldon, Harstine Island, Lake Cushman, Lilliwaup, Matlock, Potlatch, and Tahuya.