It extends approximately 442.2 miles (711.7 km) from Yellowstone National Park north to the Canadian border.

[1][2] From Bozeman, US 191 is concurrent with I-90 eastward 60.5 miles (97.4 km) to Big Timber, where it proceeds north.

The road travels through hilly ranch country near the eastern edge of the Crazy Mountains to Harlowton, where US 191 is briefly concurrent with US 12.

[1][2] US 191 continues north after going through Lewistown, crossing the Missouri River at the Charles M. Russell National Wildlife Refuge, and through Malta, where it shares a brief concurrency with US 2.

It ends at the Canada–United States border at Morgan, where the road continues into Saskatchewan as Highway 4 toward Swift Current.