Mill Mountain sits on the main Lemhi Range crest in Idaho, paired with Little Mill Mountain on a long curving ridge. The standard route climbs to Mill Lake, crosses Little Mill Mountain, and finishes up Mill Mountain's southeast ridge.
Drive to Mill Lake via Mill Creek Road (Forest Service roads FS-010 and FS-006); the last five miles are narrow, rocky, and rough, and high-clearance 4WD-Low is recommended, taking about an hour to negotiate. From the north end of the dam at Mill Lake, take the Mill Lake/East Fork Hayden Creek Trail (FST-181) to a col above the lake in 0.6 miles and 650 feet of elevation gain. From the col, follow the southwest ridge of Little Mill Mountain to that peak's summit, crossing a series of knobs that can be taken on the ridge crest or contoured on the north side.
Descend northwest off the summit of Little Mill Mountain to the connecting saddle, then ascend Mill Mountain's southeast ridge to the true summit; reverse the same line to descend.