Lewis County is located in the southwest portion of Washington State. It is bordered on the west by Pacific County, the northwest by Thurston County, the northeast by Pierce County, the east by Yakima County, the southeast by Skamania County and the southwest by Cowlitz County. As of the 2010 U.S. Census, Lewis County had a total land area of 2,402.80 square miles and a population of 75,455. Centralia is the largest city with a population of 16,336. Also of interest:
- Intersecting watershed sub-basins(10): Naches, Klickitat, Lower Columbia-Clatskanie, Upper Cowlitz, Lower Cowlitz, Lower Columbia, Upper Chehalis, Willapa Bay, Nisqually and Deschutes.
- Major lakes: Mayfield Lake and Riffe Lake.
- Major rivers: Chehalis River, Cowlitz River, Skookumchuck River, Tilton River and Cispus River.
- Big Horn (8,000 feet) is part of the Cascade Range and the highest point in Lewis County.