Dayton is a city in and the county seat of Columbia County, Washington, United States. The population was 2,448 at the 2020 census. Dayto…Dayton is a city in and the county seat of Columbia County, Washington, United States. The population was 2,448 at the 2020 census. Dayton was founded in the 1860s. A town site plat was filed by Jesse N. and Elizabeth Day on November 23, 1871. The city was officially incorporated on November 10, 1881, and was named for Jesse Day.