Leonardtown is a town in and the county seat of St. Mary's County, Maryland, United States. The population was 4,563 at the 2020 census. Historic Leonardtown includes both a large public high school and a public middle school Leonardtown Middle School as well as a Catholic high school and an elementary school Leonardtown Elementary School, offices of the county government, and St. Mary's Hospital which serves the healthcare needs of the county.