St. Lawrence Islands National Park is located in the Thousand Islands Region of the Saint Lawrence River. The islands are actually the worn-down tops of ancient mountains. This region, the Frontenac Axis, connects the Canadian Shield from Algonquin Park in Ontario to the Adirondack Mountains in New York.
The park consists of 21 islands plus many smaller islets, 2 mainland properties and a visitor centre at Mallorytown, Ontario on the mainland. This is Canada's third smallest national park with a total area of 24.4 km².