The park is situated in the Town of Alexandria on the east end of Wellesley Island, and is accessible only by boat.

Along with Canoe-Picnic Point State Park, it was one of the first New York state parks established along the St. Lawrence River as part of the St. Lawrence Reservation, a recreation area within the Thousand Islands region authorized by New York State in 1896.

[2] Mary Island State Park is accessible only by boat.

The park offers picnic tables, fishing, a boat launch and docks, and a campground for tents only.

This article about a location in Jefferson County, New York is a stub.