Fossils from Rusinga Island, Kenya, provide an exceptional collection for understanding the evolution and diversification of Early Miocene mammals in Africa, preserving rich assemblages distributed across nearly 2 million years and a variety of habitats. Placing these mammal communities within clear ecological contexts is crucial to understanding the selection pressures that shaped individual taxa but also community composition through time. Although numerous paleoenvironmental reconstructions for these deposits have been suggested, they present conflicting interpretations, with reconstructed habitats for various lithostratigraphic units and/or fossil localities ranging from closed rainforests to open and semiarid environments. Here we present the first paleoenvironmental reconstructions based on carbon and oxygen isotope composition of fossil mammal tooth enamel from Rusinga Island's important fossil assemblages, focusing on specimens from the fossiliferous Hiwegi and Kulu Formations. Our results indicate that these Early Miocene mammals foraged on plants using the C3 photosynthetic pathway in a range of habitats, including those experiencing light and/or water stress, and that habitats were differentially exploited by various species. Although the presence of a dense closed-canopy forest has been documented from at least one Hiwegi stratum, specimens sampled for our analysis exhibit only rare isotopic evidence for foraging within that habitat. Additionally, this study documents a significant difference in the oxygen isotope composition of meteoric water available to fauna found in the Hiwegi Formation versus the younger Kulu Formation. This signals a change in moisture source and/or transport path, or a climatic shift including an increase in mean annual temperature, an increase in aridity, and/or a decrease in mean annual precipitation from the Hiwegi to the Kulu Formation. These results suggest marked differences in paleoenvironmental conditions experienced by these two Early Miocene mammalian communities and underscore the need for more detailed and rigorous analyses of their taxonomic compositions, species abundances, and ecological structures.