Sumner is a city in Bremer County, Iowa, United States. The city is located along the county's eastern border, between Bremer and Fayette counties. The population was 2,028 at the 2010 census. The Bremer County portion of Sumner is part of the Waterloo–Cedar Falls Metropolitan Statistical Area. The community is named after American political leader Charles Sumner. Sumner is home to LifeLine Ambulance, a major manufacturer of ambulances around the country.