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20th century

The Great Toronto Fire of 1904 destroyed a large section of downtown Toronto. The fire destroyed more than 100 buildings. The fire claimed one victim, John Croft, who was an explosive expert clearing the ruins from the fire. It caused CA$10,387,000 in damage (roughly CA$277,600,000 in 2020 terms).

The city received new European immigrant groups from the late 19th century into the early 20th century, particularly Germans, French, Italians, and Jews. They were soon followed by Russians, Poles, and other Eastern European nations, in addition to the Chinese entering from the West. Like the Irish before them, many of these migrants lived in overcrowded shanty-type slums, such as " the Ward ", which was centred on Bay Street, now the heart of the country's Financial District.

As new migrants began to prosper, they moved to better housing in other areas, in what is now understood to be succession waves of settlement. Despite its fast-paced growth, by the 1920s, Toronto's population and economic importance in Canada remained second to the much longer established Montreal, Quebec. However, by 1934, the Toronto Stock Exchange had become the largest in the country.

In 1954, the City of Toronto and 12 surrounding municipalities were federated into a regional government known as Metropolitan Toronto. The postwar boom had resulted in rapid suburban development. It was believed a coordinated land-use strategy and shared services would provide greater efficiency for the region. The metropolitan government began to manage services that crossed municipal boundaries, including highways, police services, water and public transit. In that year, a half-century after the Great Fire of 1904, disaster struck the city again when Hurricane Hazel brought intense winds and flash flooding. In the Toronto area, 81 people were killed, nearly 1,900 families were left homeless, and the hurricane caused more than CA$25 million in damage.

In 1967, the seven smallest municipalities of Metropolitan Toronto were merged with larger neighbours, resulting in a six-municipality configuration that included the former city of Toronto and the surrounding municipalities of East York, Etobicoke, North York, Scarborough, and York.

In the decades after World War II, refugees from war-torn Europe and Chinese job-seekers arrived, as well as construction labourers, particularly from Italy and Portugal. Toronto's population grew to more than one million in 1951 when large-scale suburbanization began and doubled to two million by 1971. Following the elimination of racially based immigration policies by the late 1960s, Toronto became a destination for immigrants from all over the world. By the 1980s, Toronto had surpassed Montreal as Canada's most populous city and chief economic hub. During this time, in part owing to the political uncertainty raised by the resurgence of the Quebec sovereignty movement, many national and multinational corporations moved their head offices from Montreal to Toronto and Western Canadian cities.

On January 1, 1998, Toronto was greatly enlarged, not through traditional annexations, but as an amalgamation of the Municipality of Metropolitan Toronto and its six lower-tier constituent municipalities: East York, Etobicoke, North York, Scarborough, York, and the original city itself. They were dissolved by an act of the Government of Ontario and formed into a single-tier City of Toronto (colloquially dubbed the " megacity "), replacing all six governments.

The merger was proposed as a cost-saving measure by the Progressive Conservative provincial government under premier Mike Harris. The announcement touched off vociferous public objections. In March 1997, a referendum in all six municipalities produced a vote of more than 3:1 against amalgamation. However, municipal governments in Canada are creatures of the provincial governments, and referendums have little to no legal effect. The Harris government could thus legally ignore the referendum results and did so in April when it tabled the City of Toronto Act. Both opposition parties held a filibuster in the provincial legislature, proposing more than 12,000 amendments that allowed residents on streets of the proposed megacity to take part in public hearings on the merger and adding historical designations to the streets. This only delayed the bill's inevitable passage, given the Progressive Conservatives' majority.

North York mayor Mel Lastman became the first "megacity" mayor, and the 62nd mayor of Toronto, with his electoral victory. Lastman gained national attention after multiple snowstorms, including the January Blizzard of 1999, dumped 118 centimetres (46 in) of snow and effectively immobilized the city. He called in the Canadian Army to aid snow removal by use of their equipment to augment police and emergency services. The move was ridiculed by some in other parts of the country, fuelled in part by what was perceived as a frivolous use of resources.