The Canadian province of Ontario has several historic counties, which are past census divisions that no longer exist today. Most historic counties either merged with other counties, or became regional municipalities or single-tier municipalities. Although counties had existed prior to 1849, after 1849 they replaced the district systems in administering local government and courts in Ontario.
The county system is used in southern, southwestern and eastern sections of the province of Ontario. There are no counties in Northern Ontario due to sparse population and a long-standing boundary dispute with the Northwest Territories (that was not resolved until 1912).
Various counties throughout Ontario were joined administratively in the 19th century. While many of these still exist today and have become relatively permanent, some have since been dissolved. For example, the former United Counties of Huron and Perth existed for only a few years in the 19th century. The United Counties of Northumberland and Durham, on the other hand, merged eight years after each one was created, and continued for 174 years up until the dissolution of Durham County on January 1, 1974.
Four of Ontario's electoral districts were also erroneously listed as counties of residence in some of Canada's first post-Confederation censuses. These did not exist as counties in the political sense, although they may be referred to as such in some historical and genealogical works because of their appearances in census data:
The Regional Municipality of Sudbury can also be considered 'historic', as it later became the City of Greater Sudbury — however, its origins are not in county government, but as a part of the still-extant Sudbury District.
The unincorporated Patricia District, comprising the portion of Northwestern Ontario which was transferred to Ontario from the Northwest Territories in 1912, existed until 1937 when it was merged into Kenora District.