Kitsap Peninsula Mason County, Washington (Wikipedia) 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.