Directly descended from the Whitby Highland Rifle Company, formed in 1827, the Ontario Regiment is one of Canada's oldest units. In April of 1860, the Whitby company was one of several Toronto area units that amalgamated to form the 2nd Battalion, Volunteer Militia Rifles, but split off again in 1862, at the time of the Trent affair, to become part of the developing militia garrison of Ontario County. The Whitby Highland Rifle Company faced fire for the first time in 1866 at the Battle of Ridgeway. Later that year, it joined with eight other Ontario County infantry companies to form the 34th Battalion of Infantry, which, by 1900, was known as the 34th (Ontario) Regiment. Members of this unit volunteered for service in the Northwest Rebellion of 1885 and the Boer War.
In September 1915, the 34th Ontario Rebiment raised the 116th Battalion, canadian Expeditionary Force, from Ontario County. Its commanding officer managed to keep the 116th together and it fought with great distinction, but its battle honours came at a high price - 474 men killed. Through the 34th, Ontario County also raised the 182nd Battalion CEF, which was broken up to reinforce other battalions.
After the First World War, the Ontario Regiment came under the patronage of Sam McLaughlin, the founder of G.M. Canada and Honorary Colonel of the Regiment. "Colonel Sam's Private Army" survived the peace because McLaughlin kept it going, sometimes even paying the soldiers. The 1936 militia reorganization transformed the Regiment into a Tank Battalion and, on 1 September 1939, it mobilized for war as the Ontario Regiment(Tank). Within six days, 594 men had volunteered for active service.
The Regiment trained in England and, as part of the 1st Canadian Armoured Brigade, landed in Sicily in July 1943 and in Italy in september 1943. After fighting through Italy, the Regiment was moved to the Nijmengen sector of Holland. During the Second World War, the Regiment earned 20 battle honours at the cost of 69 killed.
In 1973, when the last of the Mark IV Sherman thnks were retired, the Regiment became wheeled reconnaissance unit. In 1980, the Ontarios acquired the Cougar Armoured Combat Vehicle and was redesignated a cougar regiment while retaining their reconnaissance role, although fiscal restraint compelled it to reduce Recce Squadron to a seven-car troop attached to Regimental Headquarters. Today, as well as performing its Reserve functions, the Regiment supplies soldiers to United Nations missions and to Canadian Forces operations in Europe and across Canada. In September 1991, it celebrated 125 years of continuous service to Canada.
In 1966, the Ontario Regiment recieved the Freedom ot the City of Oshawa and the Freedom of the County of Ontario. In 1979, the Regiment recieved the Freedom of the Region of Durham.