Clark County is a county located in the southwestern part of the U.S. state of Washington. As of the 2010 census, the population was 425,363, making it the fifth-most populous county in Washington. The county seat and largest city is Vancouver. It was the first county to be established in Washington, and named after William Clark of the Lewis and Clark Expedition. It was created by the provisional government of Oregon Territory on August 20, 1845, and at that time covered the entire present-day state of Washington.