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

In the 1870s, the Ontario and Quebec Railway Company gained a few acres of land on the south-east corner of William Lea's property to run its railway across. The station was finished in September 1884 and named "Leaside" in honor of William Lea.

In 1892 a terminal was built in the Leaside area, and a railway line was constructed south along the Don towards Union Station.
The Canadian Northern Railway incorporated the Town of Leaside in 1913. Leaside's improvement 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 suspended due to the outbreak of World War I, however the town was an important contributor to the war effort. Heavy artillery was produced at the Leaside Munitions Company. The area was also the spot of an airfield used for the training of Canadian pilots.

In 1918 the Leaside Airfield made Canadian aviation history as the accomplishment of the first airmail flight in Canada, flying from Montreal to Toronto.

Leaside's prominence 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.