Mountlake Terrace is a city in Snohomish County, Washington, United States. It is a northern suburb of Seattle and lies north of Shoreline and Lake Forest Park, east of Edmonds, south of Lynnwood, and west of Brier. Its southern boundary runs along the King County line. Interstate 5 runs north-south through the city, and services the city via three exits. The population was 19,909 at the 2010 census.