Mason County is a county located in the U.S. state of Washington. As of the 2010 census, the population was 60,699. The county seat is at Shelton, the county's only incorporated city. Mason County has several school districts.
Mason County was formed out of King 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.