Both counties were located in approximately the same area and existed on-and-off between 1792 and 1974.

[4] It was replaced by the Regional Municipality of Durham effective January 1, 1974.

A number of villages were also incorporated as separate municipalities in the county after it was created: Port Perry in 1871, Uxbridge in 1872, Cannington in 1878 and Beaverton in 1884.

In the 20th century, the wartime town of Ajax, located in Pickering Township, was incorporated as an improvement district in 1950.

In addition, the part of Pickering Township west of the Rouge River (West Rouge and Port Union) was transferred to the Borough of Scarborough in the Municipality of Metropolitan Toronto.