The island, part of Brock Township, is approximately 1,450 acres (6 km2) in size and the land is divided mainly between wooded and wooded-swamp terrain.

Thorah as well as three other islands in Lake Simcoe were ceded by the Chippewa Indians in 1856 to the British Crown on the agreement that the land would be sold and the proceeds invested for the descendants of the ceding chiefs.

Thorah was originally used as a recreational destination in the late nineteenth century by a group of four businessmen from Toronto who came to the island to camp and fish.

The island currently (2005) has a seasonal population of 147 that include several descendants of the original group.

[1] Today the island is used by the Honey Bee Research Centre at the University of Guelph as an isolated mating station for the Buckfast Bee lines maintained by the research centre.