The pass is located on the Continental Divide between the Gros Ventre mountains on the west and the Wind River Range on the east.

The pass was historically used by Native Americans and early mountain men including the Astor Expedition in 1811 on its way west.

Reports from the fur trappers who first entered the mountains and those following mention the trail through the pass as being in long term us by the Shoshone, Bannack, Arapaho, Gros Ventre, Flat Head or Bitterroot Salish, Nez Perce, Crow and others.

Crossing over Two Ocean Pass, the trail split east and west around Terrace Mountain into the Wind River Valley at DuNoir.

[4] Wilson Price Hunt, the Astorians overland to the mouth of the Columbia reported crossing Union Pass on September 15, 1811.