Centralia is a city in Lewis County, Washington, United States. It is located along Interstate 5 near the midpoint between Seattle and Portland, O…Centralia is a city in Lewis County, Washington, United States. It is located along Interstate 5 near the midpoint between Seattle and Portland, Oregon. The city had a population of 18,183 at the 2020 census. Centralia is twinned with Chehalis, located to the south near the confluence of the Chehalis and Newaukum rivers.