The Ziegler Reservoir fossil site (ZRFS) in Colorado contains over 4000 mastodon bones that date from 140,000 to 100,000 years ago. At an elevation of ~2705 meters above sea level, ZRFS represents an alpine ecosystem dated to Marine Isotope Stage (MIS) 5. Formal descriptions of cheek teeth, mandibles, crania, and femora were completed. Statistical analyses of the upper and lower third molars, including a novel measurement of interloph(id) distances, indicate significant differences between ZRFS mastodons and Mammut pacificus, while falling within the ranges for Mammut americanum. This study agrees with the taxonomic assignment of ZRFS mastodons to Mammut americanum and not Mammut pacificus. Body mass estimates of ZRFS mastodons are between 3451 and 6244 kg, and a niche model indicates elevation and water availability influenced Mammut distribution during MIS 5. Incorporating ZRFS mastodons into large comparative datasets will contribute to ongoing research into Late Pleistocene Mammut.