Arlington is a city in northern Snohomish County, Washington, United States, part of the Seattle metropolitan area. The city lies on the Stillaguamish River in the western foothills of the Cascade Range, adjacent to the city of Marysville. It is approximately 10 miles north of Everett, the county seat, and 40 miles north of Seattle, the state's largest city. As of the 2020 U.S. census, Arlington had a population of 19,868; its estimated population is 20,075 as of 2021.