Around the middle of the 19th century, the Addington Road was built by the province to encourage settlement in the northern sections of the county.

[3] In 1798, the Parliament of Upper Canada passed legislation to provide, that, at the beginning of 1800: ... the townships of Ernest Town, Fredericksburg, Adolphustown, Richmond, Camden (distinguished by being called Camden East), Amherst Island and Sheffield, do constitute and form the incorporated counties of Lenox and Addington.

[5] In 1845, the counties regained their separate identities, but still remained united for electoral purposes.

[7] 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.

[11] In the late 1990s, the County's municipalities were reorganized to form the town of Greater Napanee and the townships of Addington Highlands, Loyalist, and Stone Mills.