It was named after Charles Lennox, 3rd Duke of Richmond.
Its territory is the same as what now composes the town of Greater Napanee.
The county of Lennox, 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 Addington, on the south and west by the Bay of Quinte, to the easternmost boundary of the Mohawk village, thence by a line running along the westernmost boundary of the late township of Richmond, running north sixteen degrees west to the depth of twelve miles, thence running north seventy-four degrees east until it meets the northwesternmost boundary of the county of Addington, and comprehending all the islands in the bays and nearest to the shores thereof.
[3] The counties would regain their separate identities in 1845, but would continue to be united for electoral purposes.
[4] 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.