Along with county sub-divisions, the province also has administrative districts, hence the name of this category is Counties and Districts. Parry Sound District is an example of a district. .
At the time of this writing, Ontario had 39 remaining county or district sub-divisions. Through more recent reorganization and in a few instances of amalgamation, a number of Ontario counties became the parents of newer administrative areas known as Regional Municipalities. For example, The Regional Municipality of York has its roots in York County, while The Regional Municipality of Waterloo is the end product of Waterloo County. In a few cases, counties were combined or united as in the case of Stormont, Dundas and Glengarry Counties are considered a single united administrative district.
The names of such recently altered counties remain in this category for historical reference. The changes are recent occurrences, and many people still think in terms of the original county, rather than the newer locality, or maybe even of both. Still they are linked directly to the newer locality.
The following list includes counties which no longer exist in their original form, but instead in a Regional Municipality, or other context. They are the Counties of Dundas, Durhham, Glengarry, Haldimand, Halton, Kent, Lincoln, Peel, Victoria, Waterloo, Welland, Wentworth and York.