Highway 518 spans 72.7 kilometres (45.2 mi) between Parry Sound and Kearney.

The highway was assumed in 1956, and has remained generally unchanged since, aside from being truncated slightly at both ends.

However, the only places of noteworthy size are the village of Magnetawan and the town of Burk's Falls.

It is often used to access Grundy Lake Provincial Park, a popular camping area for northbound travellers.

It is a short and lightly travelled route that connects Highway 69 with the community of Britt.

Located in the Parry Sound District, the highway provides access to Novar, Emsdale, and others.

Located in the Parry Sound District, the highway extends for 4.4 kilometres (2.7 mi) from a junction with Lake Joseph Road, the former route of Highway 69, outside of Gordon Bay to the boundary of Parry Sound District with the regional municipality of Muskoka near a junction with Healey Lake Road.

At the boundary, the roadway continues southward as Muskoka Road 7 through the communities of Minett, Gregory and Port Sandfield.

One week later, on July 29, another 16.1 kilometres (10.0 mi) within what was then just Muskoka District was assumed as part of the highway.

It is a very minor and extremely short route, and holds the distinction of being Ontario's shortest posted highway at only 800 metres (1/2 mile) in length.

It is located in Pointe au Baril in Parry Sound District and simply acts as an access road to the community from Highway 69.