Its territory is now distributed among the townships of Addington Highlands, Stone Mills, and Loyalist.
The county of Addington, situated within the Mecklenburg District, was originally created as an electoral district for the Legislative Assembly of Upper Canada in 1792 and its original limits were described as being: bounded on the east by the westernmost line of the county of Frontenac, on the south by lake Ontario, to the westernmost boundary of the late township of Ernestown, and on the west by the easternmost boundary of the township of Fredericksburgh, running north thirty-one degrees west until it meets the Ottawa or Grand River, thence descending the said river until it meets the northwesternmost boundary of the said county of Frontenac, comprehending within the said county all the islands nearest to it, in the whole or greater part fronting the same.
[3] The counties would regain their separate identities in 1845, but would continue to be united for electoral purposes.
[4] The newly surveyed township of Anglesea was added to Addington at that time.
[5] At the beginning of 1850, Midland District was abolished, and the United Counties of Frontenac, Lennox and Addington replaced it for municipal and judicial purposes.