Highway 103 followed much of the route that Highway 400 now takes between Highway 12 at Coldwater and the former Highway 69 junction (Exit 189) south of Mactier. From there it followed what is now the east-west section of Lake Joseph Road to Foot's Bay, where it ended what was Highway 69, later Highway 169, and is presently Muskoka District Road 169 and the north-south section of Lake Joseph Road. Between Highway 400 and Foot's Bay, it is now designated as both Lake Joseph Road and Muskoka District Road 169.^[4][5] Between Coldwater and Waubaushene, the route was concurrent with Highway 12 for a brief period in the mid-1960s.^[1] From there, it crossed over Matchedash Bay and circled around the south and eastern shoreline of Sturgeon Bay to Port Severn. North of Port Severn, the route travelled through wilderness within the District of Muskoka for 38 kilometres (24 mi). At Foot's Bay, it encountered Highway 69, which continued east to Highway 11 in Gravenhurst and north to Parry Sound and Sudbury.^[4]

Highway 103 was first designated during World War II as a stub route of Highway 12 from Waubaushene to Port Severn. The 10.3 kilometres (6.4 mi) highway was assumed on July 19, 1944.^[2] The original routing until the late 1950s followed what is now Pine Street, Coldwater Road and Duck Bay Road through Waubaushene. After crossing Matchedash Bay, the route turned onto Quarry Road, then turned north and followed what is now the northbound lanes of Highway 400 straight into Port Severn. It turned west along Port Severn Road across the Trent–Severn Waterway and ended at Lone Pine Road. The entire route was gravel surfaced.^[6][7] Beginning in 1956, Highway 501 continued west to Honey Harbour.^[8]

When Ontario signed the Trans-Canada Highway Agreement on April 25, 1950, it had already chosen a Central Ontario routing via Waubaushene and Parry Sound;^[9] Highway 17 through the Ottawa Valley was announced as a provincially-funded secondary route of the TCH the following day.^[10] The route of Highway 103 was chosen as a jump-off point to connect the two places. Tenders for building the approximately 40-kilometre (25 mi) extension from Port Severn to Highway 69 at Foot's Bay were called on September 25, 1953, and included bypasses of Waubaushene and Port Severn.^[11] Construction was underway by the following year.^[12] The new paved Highway 103 was open by September 1958,^[13] although the complex three-bridge Port Severn Bypass did not open until the following spring.^[14] The old portions of the route through Waubaushene and Port Severn were transferred to Tay Township on January 29, 1959.^[15]

In order to provide better route continuity for motorists travelling from Toronto to Sudbury, several highways were renumbered in the Muskoka area on May 15, 1976. The section of Highway 69 between Foot's Bay and its southern terminus of Highway 12 at Brechin was redesignated as Highway 169, while the entirety of Highway 103 was renumbered to form the new southern portion of Highway 69.^[3] The entirety of the former route (with the exception of the east-west section of Lake Joseph Road/Muskoka Road 169 west of Foot's Bay) has now been superseded by Highway 400.^[5]

The following table lists the major junctions along Highway 103, as noted by the Ontario Department of Highways.^[1] The route is documented as it existed in 1968. 

• Approximate former route of Highway 103 on Google Maps. Note that this routing is not exact; in several locations, the highway followed roads which are now residential or local in nature and cannot be directly accessed from the current Highway 400 route.