Chester Stock in 1936 described Harrington's extinct mountain goat, Oreamnos harringtoni, based upon six skeletal elements recovered from Smith Creek Cave, Nevada. Until recently it was rarely encountered in fossil deposits of western North America and was inadequately understood. One hundred ten skeletal elements recovered from eight dry cave and wood rat midden deposits in the Grand Canyon, Arizona, enable a re-examination and redescription of the extinct species. Characters of the skull indicate that O. harringtoni was distinct from, yet similar to O. americanus, the living form. The extinct species was generally smaller overall than O. americanus, with proportionally more robust jaws. Occasionally, the fossil forms are as large as the extant species. Preserved keratinous horn sheaths and large cuboid dung pellets assigned to O. harringtoni provide carbon isotope ages directly on the extinct species. The youngest age determined on horn sheaths is 12,580±520 B.P., while the youngest age from large cuboid dung pellets is 10,870±200 B.P., both from Stanton's Cave. Hair assigned to the extinct species indicates that it had a white coat. Plant fragments in the dung indicate it ate predominantly grasses, but it also browsed heavily on Ceonothus-Cercocarpus, Prunus, Pseudotsuga, and Sphaeralcea. Oreamnos harringtoni appears to have been restricted to the Great Basin-Intermountain Region, and evolved from an ancestral population of O. americanus since the Sangomonian, in less than 100,000 years. The species became extinct by approximately 11,000 B.P.