The county is part of the Southern Maryland region.
The settlement of Lord Baltimore's Maryland began with the arrival of passengers from England at St. Clement's Island in the Potomac River in what is now southwestern St. Mary's County on March 25, 1634 (the anniversary of this landing being annually celebrated as Maryland Day). The passengers arrived in two vessels, the Ark and the Dove, which had set sail from the Isle of Wight on November 22, 1633. The county is also home to the first Catholic Mass celebrated in one of the original thirteen colonies -after they'd become English colonies (masses were said in 1526-7 at San Miguel de Gualdape).
St. Mary's County was the first county established in Maryland, in 1637, probably by an order of the Governor.
The county seat and state capital was St. Mary's City.
In 1649, Lord Baltimore, with the Maryland General Assembly, passed the Maryland Toleration Act, which provided religious freedom for any (Christian) sect, and which was the first law of its kind in the New World. There is a statue in St. Mary's City commemorating this event.
St. Mary's County is where Francis Scott Key, the author of a poem which became The Star Spangled Banner, and professional wrestler Scott Hall grew up. St. Mary's County was the birthplace of Dashiell Hammett, and Orlando "Tubby" Smith, head basketball coach at the University of Minnesota. The largest employer is Patuxent River Naval Air Station and its related aerospace contract firms. There are both Amish and Mennonite communities who follow traditional ways. Tobacco, once dominant crop, has declined in recent years. The Maryland International Raceway (Budd's Creek) attracts many auto racing enthusiasts.
The county has a number of properties on the National Register of Historic Places.
Saint Mary's County Families by Linda Reno Davis and Marcella Jehl Dawson is extremely helpful to researchers of early Saint Mary's county. It features abstracts of wills from 1638-1800, marriages from 1638-1820, and numerous other articles and projects.
The Saint Mary's County Genealogical Society offers memberships and publishes a newsletter called The Generator four times a year.