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 United Kingdom's 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.

en.wikipedia.org/wiki/Flag_of_Ontario