Simcoe County, in particular the former Wendake area near Nottawasaga Bay, was the site of the earliest French exploration and settlement of Ontario; they were the first Europeans in the area. Several historic sites, including Carhagouha and Sainte-Marie among the Hurons, mark the earliest known contacts between the area's traditional Huron population and French missionaries. The Huron capital, Ossossané, was at one time the largest aboriginal settlement in all of North America outside of Mexico

The area was established by British Army officer and Lieutenant Governor of Upper Canada John Graves Simcoe as "Simcoe District", in honour of his father Captain John Simcoe, in 1843 by the Legislative Assembly of Upper Canada. Its original 19 townships at that time were: Adjala, Essa, Flos, West Gwillimbury, Innisfil, Mara, Matchedash, Medonte, Nottawasaga, Orillia (North Division), Orillia (South Division), Oro, Rama, Sunnidale, Tay, Tecumseth, Tiny, Tosorontio and Vespra.

The District was restructured in 1845, changing its composition to the following 24 townships (Source: Statutory Chronology of Canada): Adjala, Artemesia, Collingwood, Essa, Flos, West Gwillimbury, Innisfil, Medonte, Matchedash, Mulmur, Mono, Nottawasaga, Osprey, Oro, North Orillia, South Orillia, Saint Vincent, Sunnidale, Tay, Tecumseth, Tosorontio, Tiny, Uphrasia (sic) and Vespra.

Effective January 1, 1850, An Act for abolishing the Territorial Divisions of Upper Canada into Districts abolished Simcoe "District" in favour of the Simcoe "County", still composed of the Townships noted in the Act of 1845.

The most recent restructuring took place in 1994 which resulted in the current 16 local municipalities.