Lewis County is a county in the U.S. state of Washington. As of the 2010 census, the county's population was 75,455. The county seat is Chehalis, and its largest city is Centralia. The county was created as Vancouver County on December 19, 1845, by the Provisional Government of Oregon, named for George Vancouver. In 1849, the county name was changed, to honor Meriwether Lewis. At the time, the county included all U.S. lands north of the Cowlitz River, including much of the Puget Sound region and British Columbia.