Mahtomedi is a city in Washington County, Minnesota, United States. The population was 7,676 at the 2010 census. Mahtomedi is a suburb of …Mahtomedi is a city in Washington County, Minnesota, United States. The population was 7,676 at the 2010 census. Mahtomedi is a suburb of St. Paul and is located between St. Paul and Stillwater along the shores of White Bear Lake.