Before 1965, the Canadian Red Ensign had served as the de facto national flag of Canada. It was flown at all military installations in Canada and overseas, outside the legislature and government buildings, at Royal Canadian Legion halls, and many private homes.
In 1964, the federal government, after a long and acrimonious debate, replaced the Red Ensign with the current flag of Canada. This decision was unpopular among some Canadians. These included many Ontarians, particularly in rural areas that made up much of the political base of Premier John Robarts' Ontario Progressive Conservatives.
Robarts thus proposed that Ontario would have its own flag and that it would be a Red Ensign like the previous Canadian flag. It was traditional for jurisdictions around the world with a British system of government and way of life to adapt either a blue or red ensign as a flag, by adding the local coat of arms or some other symbol. In Ontario, it was logical to place the Ontario shield of arms on the flag. Robarts felt the Ensign was an important symbol that reflected Ontario's British heritage and the sacrifices made by Canadian troops under the Red Ensign. As Robarts put it, the Ontario flag "covers our history".
Canadians were exhausted by the long debate over the national flag and leaders of both the Ontario Liberal Party and the Ontario New Democratic Party supported the design for the proposed Ontario flag. The only opposition came from Sudbury Liberal Member of Provincial Parliament Elmer Sopha who was fervently opposed to the flag, arguing that it failed to reflect Ontario's diverse character and that it was "a flag of revenge" against the new national flag. However, he was joined by only one other MPP, Liberal Leo Troy, in voting against the flag, and it was passed by the Legislative Assembly on March 17. It went in effect on May 21, 1965.
The flag of Manitoba was adopted under similar circumstances.