It begins at an intersection with U.S. Highway 191 (US 191) in far southwestern Phillips County and runs northerly to the town of Fort Belknap in Blaine County.

Approximately 40 miles (64 km) of the northern end of the route is within the boundaries of the Fort Belknap Indian Reservation,[1]: 193  running through the bulk of its territory.

MT 66 starts at an intersection with US 191 approximately 55 miles (89 km) southwest of Malta.

[1]: 88–89  It proceeds in a generally northwesterly direction, skirting the western edge of the Little Rocky Mountains.

After passing the small community of Hays on its western edge, the road continues north on a mostly straight route before turning northwest and enters Fort Belknap from the south, before a final turn to the northeast joins MT 66 to US 2 at a rest area.