It connects U.S. Route 93 (US 93) south of Whitefish to US 2 west of Columbia Falls.

MT 40 begins at US 93 on the southern outskirts of Whitefish and proceeds eastward in a roughly straight line to its terminus with US 2 west of Columbia Falls.

MT 40 forms the central part of the most direct link between the two cities, allowing travel from the Hi-Line communities and Glacier National Park to the communities of the Columbia Valley in British Columbia without needing to detour south via Kalispell or north via the Crowsnest Pass between British Columbia and Alberta.

[2] For a brief time in the 1940s, it was even signed as MT 37, overlaid on US 93 to north of Eureka and then going to Libby, as seen on the 1942 map.

[4] MT 40 extended through Columbia Falls to the current US 2/Secondary Highway 206 junction until the US 2 and S-206 routings were swapped in 1983.