Saint Mary's is one of the 24 counties in Maryland. It was formed in 1637, and was the original county formed in Maryland. As of the 2010 census, the population was 105,151, up from 86,211 in 2000. The county seat and largest city is Leonardtown.
The county contains 764 square miles (1,979 km2), of which only 361 square miles (935 km2) is dry land. It is bordered by Charles County on the northwest, Calvert County to the northeast, Chesapeake Bay on the east and the Potomac River to the west.