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.

St. Mary's County, established in 1637, is a county located in the U.S. state of Maryland. As of the 2020 census, the population was 113,777. Its county seat is Leonardtown. The name is in honor of Mary, the mother of Jesus. St. Mary's County comprises the California-Lexington Park, Maryland Metropolitan Statistical Area, which also is included in the Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area. It is part of the Southern Maryland region. The county was the home to the first Maryland Colony, and the first capital of the Colony of Maryland. Settled by English Catholics, it is considered to be the birthplace of religious freedom in North America, at a time when the British colonies were settled primarily by Protestants. The county is home to the Patuxent River Naval Air Station and St. Mary's College of Maryland. Traditionally, St. Mary's County has been known for its unique and historic culture of Chesapeake Bay tidewater farming, fishing, and crabbing communities. But with the advent of the military bases, growth of an extensive defense contractor presence, and the growth of St. Mary's College of Maryland, as well as increasing numbers of long-distance Washington, D.C. commuters, it has been undergoing a decades-long transformation which has seen the county's population double since 1970.