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.

• Addington County merged with Lennox County to form Lennox and Addington County
• Brant County split into the single-tier 'county' of Brant and the city of Brantford
• Carleton County became the Regional Municipality of Ottawa-Carleton, now the City of Ottawa, a single-tier municipality.
• Dundas County merged with Stormont and Glengarry Counties to form Stormont, Dundas and Glengarry United Counties
• Durham County portions merged with Ontario County to form Durham Regional Municipality. Remaining portions joined Northumberland and Victoria Counties.
• Frontenac County was abolished and replaced with the Frontenac Management Unit. Informally, the area is still known as Frontenac County, even though it isn't officially one.
• Glengarry County merged with Stormont and Dundas Counties to form Stormont, Dundas and Glengarry United Counties
• Grenville County merged with Leeds County to form Leeds and Grenville United Counties
• Kent County became the single-tier Municipality of Chatham-Kent
• Haldimand County was merged into the Regional Municipality of Haldimand-Norfolk for many years, but this was divided again on the old lines in 2001; it is now a single-tier municipality, not an official county
• Halton County became Halton Regional Municipality
• Leeds County merged with Grenville County to form Leeds and Grenville United Counties
• Lennox County merged with Addington County to form Lennox and Addington County
• Lincoln County merged with Welland County to form Niagara Regional Municipality
• Norfolk County was merged into the Regional Municipality of Haldimand-Norfolk for many years, but this was divided again on the old lines in 2001; it is now a single-tier municipality, not an official county
• Ontario County originally existed from 1792 to 1800, and was split with the eastern portion joining Frontenac County and the western portion joining Addington County. The second existed from 1858 to 1973, and merged with portions of Durham County to form Durham Regional Municipality
• Peel County became Peel Regional Municipality
• Prescott County merged with Russell County to form Prescott and Russell United Counties
• Prince Edward County has retained its name, but is now a single-tier municipality, not an official county
• Russell County merged with Prescott County to form Prescott and Russell United Counties
• Stormont County merged with Dundas and Glengarry Counties to form Stormont, Dundas and Glengarry United Counties
• Suffolk County was split into the Counties of Kent and Middlesex.
• Victoria County became the City of Kawartha Lakes, a single-tier municipality.
• Welland County merged with Lincoln County to form Niagara Regional Municipality
• Waterloo County became Waterloo Regional Municipality
• Wentworth County became Hamilton-Wentworth Regional Municipality, now the City of Hamilton, a single-tier municipality.
• York County saw Metropolitan Toronto (now the City of Toronto) separate from it in 1953, and the remainder became York Regional Municipality in 1971

Various counties throughout Ontario formed political alliances in the 19th century. While many of these alliances still exist today and have become relatively permanent, some have since been dissolved. For example, the former United Counties of Huron and Perth only existed for a few years in the 1800s. The United Counties of Northumberland and Durham, on the other hand, merged eight years after each one was created, and continued to co-exist for 101 years right up until the dissolution of Durham County in 1973.

Ontario also has four historic counties which appeared in some census enumerations and as electoral ridings, but never existed as separate political entities:

• Bothwell County was made up of townships from Kent and Lambton counties.
• Cardwell County was made up of townships from Simcoe and Peel counties.
• Monck County was made up of townships from Lincoln, Haldimand, and Welland Counties.
• Niagara County was made up of townships from Lincoln County.

The Regional Municipality of Sudbury can 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.

See also List of Ontario census divisions.

External links

• Maps of historic counties