High Quality Content by WIKIPEDIA articles! Kelso is a city in southwest Washington State, United States, and is the county seat of Cowlitz County. At the 2010 census, the population was 11,925. Kelso is part of the Longview, Washington Metropolitan Statistical Area, which has a population of 102,410. Kelso shares its long western border with Longview, Washington. It is located near Mount St. Helens.