Highway 66 passes through the controversially-named community of Swastika
Beginning at the village of Matachewan, where the highway continues west as Highway 566, the route travels 4.6 kilometres (2.9 mi) east to a junction with Highway 65. From there to the community of Kenogami Lake, on Highway 11, the highway passes through a 40 kilometres (25 mi) wilderness, encountering few roads or signs of humanity. Instead the highway winds through rock cuts, muskeg and thick coniferous forests. After intersecting Highway 11, the route continues east through the community of Swastika. It encounters Highway 112 between Swastika and the community of Chaput Hughes, after which the highway enters the town of Kirkland Lake. East of the town, Highway 66 passes through King Kirkland and encounters Highway 672. East of here, the highway returns to a remote setting, passing through the community of Larder Lake, where it encounters Highway 624. For the remaining 17 kilometres (11 mi), the route snakes through the wilderness, passing through the communities of Virginiatown and Kearns between long segments of forest. Immediately east of Kearns, the highway crosses the Ontario–Quebec boundary, where it continues as Quebec Route 117 to Rouyn-Noranda.
The entirety of Highway 66 is located within Timiskaming District in the rugged and remote Canadian Shield. Outside of the communities along the route, there is almost no inhabitation or services. Consequently, traffic volumes drop considerably east of Highway 11.
Highway 66 was first assumed by the Department of Highways on September 22, 1937, shortly after its merger with the Department of Northern Development (DND). The DND created the road during the early 1930s, connecting several rail stops. The Kirkland Lake area is the site of several gold deposits that were discovered during the first quarter of the century, and the King's Highway status brought about new improvements to help service the mines. Initially, the route was only 26.2 miles (42.2 km) long, connecting Kirkland Lake with the Ontario–Quebec boundary. On November 16, 1955, the route was extended 25.5 miles (41.0 km) west to Highway 65 near Matachewan. Although several minor realignments to improve the rugged route have been made since then, it did not change significantly between 1956 and 1997. On April 1, 1997, a 1.9 kilometres (1.2 mi) section of the highway, between Goldthorpe Drive and Main Street, was transferred to the town of Kirkland Lake.