Several decades (1940s–2011) of prospecting and collecting of mammalian fossils from late Eocene strata of the White River Formation in Kings Canyon, a high-altitude (∼2,500 m; 8,200 ft) paleovalley incised into the Medicine Bow Mountains near the Wyoming–Colorado border, have produced a significant faunal assemblage that includes at least 14 species (in 13 families). Research by others suggests that Kings Canyon was at least at its present-day elevation during late Eocene time. Study of the fossil vertebrates, therefore, provides a rare glimpse into a late Eocene, high-altitude mammalian fauna. Here, we describe the Kings Canyon fauna that includes: the lagomorph Megalagus; the rodents Ischyromys, Cylindrodon, Pelycomys, and Paradjidaumo; the carnivore Hesperocyon; the artiodactyls Archaeotherium, Bathygenys, Poabromylus, Pseudoprotoceras, and Leptomeryx; and the perissodactyls Mesohippus, Megacerops, and Rhinocerotidae. The mammalian fauna corroborates the Chadronian (late Eocene) age of the White River Formation in Kings Canyon suggested by others. The co-occurrence of Megalagus brachyodon, Leptomeryx speciosus, and Pseudoprotoceras longinaris suggests a middle to late Chadronian age. Several taxa reported here represent geographic range extensions. Leptomeryx speciosus and Poabromylus are extended south from Wyoming, while the discovery of Bathygenys alpha at Kings Canyon represents the first known occurrence of this species in Colorado (otherwise known from Wyoming, Montana, and Texas). Ranges of Cylindrodon, Hesperocyon, and Archaeotherium are extended slightly westward from northeastern Colorado. Similar to the late Chadronian-aged Florissant fauna from central Colorado, geographically and faunally the Kings Canyon fauna appears to be near the border between the Great Plains and Rocky Mountain Provinces defined by others. Also, like Florissant, the Kings Canyon fauna seems consistent with weakened faunal provinciality during Chadronian time.