US 12's through Montana's mountains and plains is the greatest distance that US 12 traverses through any state. The highway enters Montana at Lolo Pass, southwest of Lolo Hot Springs in the Lolo National Forest. After passing Lolo Peak to the south and traveling east for , it meets with US 93 at Lolo and continues as a concurrency northeast for , where US 93 heads due north on Reserve Street, toward Glacier National Park. US 12 continues northeast through Missoula's downtown, eventually meeting I-90. It then overlaps I-90 for , until Garrison, where it heads east toward Helena for . This two-lane section of the trip passes through Avon and Elliston winding through the Helena National Forest, over the Continental Divide at MacDonald Pass, and then through Montana's capital city, Helena. US 12 passes over Interstate 15 at which, point it joins US 287 south. US 12 overlaps US 287 and heads southeast, toward Townsend for , where it splits from US 287, which heads south for toward the intersection of I-90 near the town of Three Forks. US 12 heads east toward White Sulphur Springs for . The route joins US 89 for before entering White Sulphur Springs, and for another east of town. US 89 splits north and US 12 continues east on its own for , until the junction with I-94 at Forsyth as a concurrency northeast for , to Miles City. At the east exit for Miles City, US 12 splits again from I-94 and heads almost directly east to the North Dakota border at a distance of .