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. Leonardtown is perhaps most famous for the national oy…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. Leonardtown is perhaps most famous for the national oyster-shucking championship that is held annually at the St. Mary's County fairgrounds.