Grand Lake covers 13,500 acres (5,500 ha) in Mercer and Auglaize counties. Grand Lake is the largest inland lake in Ohio in terms of area, but is very shallow, with an average depth of only 5 to 7 feet (1.5 to 2.1 m).
Grand Lake St. Marys was constructed in the early 19th century as a reservoir for the Miami and Erie Canal, which connected the Ohio River with Lake Erie. At one time the lake was the world's largest man-made lake. The canal system thrived for about thirty years in the mid-to-late 19th century before it was replaced by the railroads. In the 1890s oil was discovered in the area, and oil derricks were set up in the lake. The lake became one of Ohio's first state parks in 1949 with recreational facilities built in the following years.
The official geographic name of the lake is Grand Lake. The United States Geological Survey Board of Geographic Names disapproved a proposal to change the name Grand Lake to Grand Lake-Saint Marys, which is the name given on state maps and documents. The 1920 edition of the Encyclopædia Britannica shows the name as Lake Mercer on the map of Ohio in the article on Ohio.
Grand Lake St. Marys State Park is open for year-round recreation, including boating, hunting, fishing, swimming and picnicking. The campground has 216 sites of which 135 are equipped with electricity. Facilities include modern restroom facilities, a laundry and a dump station. Pets are permitted. On the lake near the camping area there is a “camper’s only” swimming beach and a boat dock and launch. Two small and three large cabins are available to rent May–October. Two modern cabins are available for daily or weekly rental.
Grand Lake St. Marys is open to boating, fishing, swimming and water skiing. Boats with unlimited horsepower are permitted on the lake. The state maintains five launch ramps provide access to the lake. A 300-foot (91 m) no wake zone has been established around the lake’s 52 miles (84 km) of shore. Boats are prohibited from entering a wildlife refuge on the southwest corner of the lake.
The lake is open to fishing year-round. Common game fish include warm water fish such as crappie, bass, yellow perch, and bluegill. Hunting is permitted in designated zones. Grand Lake St. Marys provides a habitat for a variety of waterfowl. The state owns ninety duck blinds spread throughout the designated hunting zones.