Highway 15 was one of the original highways created by the Department of Public Highways, the early predecessor to the Ministry of Transportation of Ontario (MTO). On October 13, 1920, several dirt roads through Leeds and Grenville County and Lanark County were designated as part of the expanding provincial highway network begun a year earlier. The route began in the community of Seeley's Bay and travelled along the current routing of the highway to Smiths Falls. It then turned northwest towards Perth. At Perth, the route turned northeast towards Carleton Place and followed roughly the same alignment as Highway 7 does today. The highway passed through that town and winded towards Ottawa, entering it along Richmond Road and ending in downtown at Highway 17.