Situated at the northern end of the St. Mary's River watershed, the park shows a wide range of habitats, from wooded acres and fields to swamps and small streams. With such a diversity, you can also find many different types of plant and animal species. Check out a map of the area.

St. Mary's River State Park is separated into two areas, Sites 1 and 2. The Site 1 area holds the 250 acre St. Mary's Lake. Located along Maryland Route 5, between Leonardtown and Great Mills, at the end of Camp Cosoma Road, the area has become a popular freshwater fishing spot. Several species of fish are common to the lake including largemouth bass, chain pickerel, crappie, bluegill and sunfish. The lake has been designated a trophy bass lake and as such, special fishing regulations may be in effect.

From as far back as 3000 B.C., evidence indicates that Indians lived in the Southern Maryland area. Several different tribes occupied the area at different times, some of which include the Piscataway-Conoy, Algonquins, and the war-like Susquehannocks. Many of the names they used for their villages and rivers are still used today. Items left by these people such as arrowheads, axe heads and pottery can still be found in the fields along the stream banks of the St. Mary's River.