Sir William Mackenzie (October 17 1849 – December 5 1923) was a Canadian railway contractor and entrepreneur.

Born near Peterborough, Ontario, Mackenzie became a teacher and politician before entering business as the owner of a sawmill and gristmill in Kirkfield, Ontario. He entered the railway business as a contractor, working on projects in Ontario, British Columbia, Maine, and the North-West Territories (present-day Saskatchewan and Alberta) between 1874-1891.

Mackenzie became part-owner of the Toronto Street Railway (precursor to the Toronto Transit Commission) in 1891 and in 1899, helped found the precursor to Brazilian Traction, for which he was the first chairman. In 1895, together with Donald Mann, Mackenzie began to purchase or build rail lines in the Canadian prairies which would form the Canadian Northern Railway (CNoR), a company which would stretch from Vancouver Island to Cape Breton Island and form Canada's second transcontinental railway system.

Mackenzie and Mann were knighted in 1911 for their efforts in the railway industry, but personal and company financial difficulties led to the bankruptcy of the CNoR and the system was nationalized by the federal government on September 6, 1918.

Mackenzie died in Toronto, Ontario.