We explored the stable light isotope ecology of a sample of extinct Namibian mammals from the early Miocene, a period when conditions are believed to have been relatively warm and moist. Where possible we compared isotope values of mammalian tooth enamel (Rhinocerotidae, Suidae, Climacoceratidae, Pliohyracidae, Gomphotheriidae and Deinotheriidae) with those of eggshells of a co-existing Aepyornithoid (19–20 Ma). In general, the isotopic values of tooth enamel in all mammalian fauna, including likely mixed feeders/grazers, are consistent with C3 diets as expected for this period, and the ratite eggshells are consistently enriched in both 13C and 18O compared to the mammals. High eggshell carbon and oxygen isotope values at the sites of Grillental and Elisabethfeld suggest that ratite birds occupied a dry niche that possibly included CAM plants, while results for Langental suggest moister, cooler conditions. Arrisdrift (ca. 17–17.5 Ma) mammalian tooth enamel values overlap those of the 19–20 Ma sites. The dataset for Arrisdrift reveals distinctive patterning between species. In particular the two suid species differ significantly in δ18O and δ13C, suggesting distinct ecological niches consistent with predictions from their dental morphology. On the other hand, two distinct data populations for one species of Prohyrax may suggest different environmental conditions during the period in which the site formed. Keywords: Stable isotopes, Mammalian teeth, Ratite eggshells, Namibia, Miocene.