In 1914, World War I began. The world’s most famous war poem, "In Flanders Fields," would be written the following year by Lieutenant-Colonel John McCrae, a doctor from Guelph serving in France. An influenza epidemic killed thousands in 1918 (worldwide, at least 20 million).

After the war, the economy went boom, then bust. The stock market crash of 1929 heralded the beginning of the Great Depression. In 1933, one in five Canadians was unemployed; by 1935, one in 10 Canadians was on relief. The Depression only ended with the onset of World War II in 1939. Over the next six years, one million Canadians would fight and 42,000 would be killed.

In 1943, the Progressive Conservatives were elected to lead the Legislative Assembly and would be continually re-elected for the next 42 years.

During and after the war, Canada and Ontario prospered. The population increased 40 per cent from 1945 to 1958, thanks to a "baby boom" and postwar immigration of more than 100,000 people per year through the 1950s. By 1961, immigrants accounted for one in five Ontarians.