Situated at the northern end of the St. Mary's River watershed, 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. This area has been improved by a modern comfort station, picnic tables, playground, boat launch ramps and a large gravel parking lot. Pets are allowed in the state park. Site 1 St. Mary's River State Park offers an accessible bathroom and seasonal boat launching facility. ​​Site 2 covers approximately 2,200 acres and is primarily undeveloped. It is a wildlands area and a managed hunting area. Parking is permitted in designated areas only. 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. The park shows a wide range of habitats, from wooded acres and fields to swamps and small streams.

Trails

A 7.5 mile multi-use trail circles the lake and is popular for hiking, biking and horseback riding.

The Maryland Park Service does not rent horses, and only provides the trails as a means of recreation.

Fishing and Boating

A 7.5 mile trail circles the St. Mary's Lake, allowing the area to be fished from shore or by boat (electric motor only). 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. Fishermen should check bulletin boards or contact park personnel for details. Electric motors only are allowed on the lake. A boat ramp is located on the lake.

Hunting

The 2,200 acre Site 2 is primarily undeveloped. It is a wildlands area and a managed hunting area.

Small game such as rabbit and squirrel, as well as white-tailed deer are found here. Hunting with a firearm is permitted. You may hunt with bow and arrow during the regular archery season for deer.