The flag of Ontario was enacted by the Flag Act on May 21, 1965 in the Legislature of the Province of Ontario. The flag is a defaced Red Ensign, with the Union Flag in the canton and the Ontario shield of arms in the fly.
Before 1965, the Canadian Red Ensign had served as the 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 millions of 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. While Robarts insisted that he supported the new national flag, he 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 decided to support 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 only joined by one other MPP, Liberal Leo Troy, in voting against the flag, and it was passed by the Legislative Assembly on March 17.
The flag of Manitoba was adopted under similar circumstances.
In 2001, a survey conducted by the North American Vexillological Association (NAVA) placed the Ontario provincial flag 43rd in design quality out of the 72 Canadian provincial, U.S. state, and U.S. territory flags ranked.