The settlement of the Canadian West and the tapping of northern Ontario's forests and mines opened new markets, ensuring Toronto's continued prosperity. Harnessing Niagara provided cheap hydroelectric power.

By 1912, Toronto had doubled in physical size, having absorbed surrounding villages and towns during the previous 30 years. In the booming 1920s, new suburban municipalities rose around an overflowing city of some half million people.

Thousands of Torontonians enlisted during the two world war efforts, also aided by the city's factories. Toronto's diverse economy mitigated the effects of the 1930s Depression.

Most immigrants were still from the British Isles, but increasingly from continental Europe as well. Many lived in "The Ward" behind city hall, where a small Chinatown also developed.