Afton is a city in Washington County, Minnesota, United States. The population was 2,886 at the 2010 United States Census. It lies on a s…Afton is a city in Washington County, Minnesota, United States. The population was 2,886 at the 2010 United States Census. It lies on a small bay where Valley Creek empties into the St. Croix River, several miles north of its confluence with the Mississippi River.