Mason County is located in the heart of the Puget Sound Basin in Southeast corner of the Olympic Peninsula. It is north of Thurston County, east of Grays Harbor, West of Kitsap County, and South of Jefferson County. Originally named Sahewamish County for the first ten years, it was renamed in 1864 to Mason County after the honorable Charles H. Mason, the first Washington Secretary. 

With 961 square miles (615,040 acres) of land (275 square miles are in national park or forest), and 900 miles of shoreline, Mason County represents an outdoor playground for nearly every imaginable activity. Blessed with dramatic scenery, the county's topography is marked by hills, rivers, lakes, green valleys, and thick forests. Altitude ranges from sea level to the snowfields of Mt. Washington, at 6,255 feet.

It is still relatively sparsely populated land with 58,016 inhabitants. Two-thirds of its area is covered with forest. The county has a resource-based economy dominated by government sector with 2,882 among 16 government entities. The Washington Correction Center employs 660 employees but the Squaxin Island Indian Tribe is the largest employer with 862 with its tribe's casino-hotel and tribal government.