In 1974, the two counties were split and reorganized, with the former portions of Durham County reorganized into the Regional Municipality of Durham.

Durham County was created in 1792 by a proclamation of Lieutenant Governor Simcoe.

Effective January 1, 1974, about half of Durham County was merged with Ontario County to create the Regional Municipality of Durham.

The township of Manvers was transferred to Victoria County, which is now the city of Kawartha Lakes, Cavan was transferred to Peterborough County, where it is now part of Cavan-Monaghan, and Hope was transferred to Northumberland County, where it is now part of the town of Port Hope.

The county was originally composed of the townships of Cartwright, Manvers, Cavan, Darlington, Clarke and Hope, and portions of what is now Peterborough County, created in 1838.