Stable isotopic techniques are emergent as a powerful tool for paleoecological reconstructions of Neogene terrestrial mammals. Reconstructions of herbivore diet may be augmented by correlation with paleofloral analyses. The Las Vegas Valley in southern Nevada contains a diverse Late Pleistocene fossil assemblage. Pleistocene packrat midden and pollen data for the Mojave Desert provide an independent, site-specific record of vegetation during pluvial climate. This study investigates the diets of four megafaunal genera (Mammuthus, Equus, Bison, and Camelops) using δ¹³C signatures preserved in tooth enamel. These fossils were recovered from a Late Pleistocene spring mound in the northwestern Las Vegas Valley; radiocarbon tests constrain the age of these fossils to the Last Glacial Maximum (LGM). The δ¹³C values from the three grazing genera (Mammuthus, Equus, and Bison) indicate that these animals consumed C₃ and C₄ grasses in the naturally occurring proportion, which locally consisted primarily of C₃ grasses. The δ¹³C values from Camelops indicate the highest proportion of C₄ plants consumed. Independent vegetation records, paired with paleoclimatological reconstructions, indicate a low abundance of C₄ grasses during the LGM, but a moderate abundance of the halophytic C₄ shrub Atriplex, a substantial component of modern Mojave Desert vegetation. Modern camelids prefer halophytic plants, including Atriplex. Data from this study indicate that fossil camelids in North America also demonstrated this preference for salty plants, and consumed browse material with a high proportion of C₄ halophytic shrubs. Results from serial sampling are also presented as a subannual record of dietary variation and seasonality. Intratooth δ¹³C variation is approximately 1-2 0 in grazing taxa and 3-4 0 in the browser. Some δ¹³C variation exhibits an inverse relationship with intratooth δ¹⁸O values. This study provides new insight into isotopic reconstructions of paleoecology in arid environments.