There are 9 US Post Offices in Mason County serving a total of 59,281 residents equating to 6,587 residents per post office. It's estimated that approximately 94,619 packages are handled across the 9 county post offices annually. 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 and only incorporated city is Shelton. The 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. Mason County comprises the Shelton, WA Micropolitan Statistical Area and is included in the Seattle-Tacoma, WA Combined Statistical Area."