Mason County is located on the Olympic Peninsula in Washington State. It is bordered on the west by Grays Harbor County, the northwest by Jefferson County, the northeast by Kitsap County, the east by Pierce County and the south by Thurston County. As of the 2010 U.S. Census, Mason County had a total land area of 959.42 square miles and a population of 60,699. Shelton is the largest city with a population of 9,834. Also of interest:
- Intersecting watersheds sub-basins(5): Queets-Quinalt, Lower Chehalis, Skokomish, Hood Canal and Puget Sound.
- Major lakes: Lake Cushman and Mason Lake.
- Major rivers: Union River, Tahuya River, Dewatto River, Hamma Hamma River, S. Fork Skokomish River and E. Fork Satsop River.
- Mount Washington (6,260 feet) is part of the Olympic Mountains and the highest point in Mason County.