Amherst Island is located in Lake Ontario, 10 kilometres (6 mi) west of Kingston, Ontario, Canada. Amherst Island, being wholly in Lake Ontario, is upstream, above the St Lawrence River Thousand Islands. The island is part of Loyalist Township in Lennox and Addington County. Amherst Island is located about 3 kilometres (2 mi) offshore from the rest of Loyalist Township and is serviced by public ferry from Millhaven. The Island measures over 20 kilometres (12 mi) in length from Bluff Point in the southwest to Amherst Bar in the northeast and over 7 kilometres (4.3 mi) at its widest point across. The island is about 66 square kilometres (25 sq mi) in size and is one of the largest islands in the Great Lakes.

The Thousand Islands constitute a North American archipelago of 1,864 islands that straddles the Canada–US border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for about 50 miles (80 km) downstream from Kingston, Ontario. The Canadian islands are in the province of Ontario and the U.S. islands in the state of New York.