Leaside was first settled by John Lea, a pioneer farmer who emigrated to Canada from Philadelphia in 1819. In the 1850's, Lea's oldest son William built an eight-sided octagonal shaped house - appropriately named 'Leaside' - near the present day site of Leaside Memorial Gardens. This neighbourhood has been called Leaside ever since.

The Canadian Northern Railway incorporated the Town of Leaside in 1913 on land formerly owned by the Lea family. Leaside's development was historically significant in that it was the first town in Ontario to be completely planned on paper before any homes were actually built.

Leaside's residential development was stalled due to the outbreak of World War I, however Leaside was an important contributor to the war effort. Heavy artillery was manufactured at the Leaside Munitions Company, and Leaside was also the location of an airfield used for the training of Canadian pilots.

In 1918, the Leaside Airfield made Canadian aviation history as the terminus of the first airmail flight in Canada, travelling from Montreal to Toronto. Leaside's status as a Town came to an end in 1967, when it became part of the Borough of East York, which has since amalgamated with the City of Toronto.