Mason Lake is a natural elongated lake south of Belfair, Washington, USA in Mason County. Named for Charles H. Mason, the first secretary of Washington Territory, the lake is on the isthmus of the Kitsap Peninsula between the Hood Canal and Pickering Passage of the Case Inlet. The smaller Benson Lake is to its eastern side.Mason Lake is approximately 1000 acres in size and four miles long. Its maximum depth is 90 feet and the mean depth is 48 feet.