In 1792 "The seventh of said counties be hereafter called by the name of the County of Ontario which county is to consist of the following islands, an Island at present known by the name of Isle Tonti to be called Amherst Island an Island known by the name of Isle au Fòret to be called Gage Island an Island known by the name of Isle Cauchois to be called Howe Island and to comprehend all the Islands between the mouth of the Gannoque to the Easternmost extremity of the late township of Marysburg called Point Pleasant."⁶

Amherst Island "was part of an extensive land grant made in 1675 by Louis XIV of France to explorer Sieur de La Salle, who named it Isle de Tonti after his lieutenant."²

In 1800 "the county of Ontario was abolished in favour of the counties of Frontenac and Addington."⁶

See Addington County & Frontenac County for further information on this area.

Ontario County was located in South-Eastern Ontario.