In 1850 Durham County and Northumberland County were united for municipal purposes such as road maintenance, and the delivery of services such as water and sewage. The united county had wide powers relating to the regulation of land and local administration through by-laws. In the Archives of Ontario data respecting genalogical matters such as vital statistics and land records are more likely to be filed under Northumberland and Durham or under their individual townships, than under "Northumberland and Durham".
In 1974 the counties were separated. Northumberland now contains its original boundaries but also took on Hope Township from Durham. Durham County joined with the greater part of Ontario County to its west to become the Regional Municipality of Durham.
Ontario GenWeb county pages include outline maps of the townships in the county, as well as detailed listings of the genealogical material available.