Native prairie plant communities are among the most endangered habitats due to extensive agricultural use and exotic species invasion. Understanding how species composition, particularly of exotic species, varies with environment is essential for management and preservation of these communities. In semi-arid prairies, plant establishment is often heterogeneous across the landscape and depends on the patchy availability of resources across environmental gradients. We studied how soil geologic substrate, topography, and grazing history relate to patterns of native and exotic plant distribution in a semiarid Mima mound prairie in eastern Washington. We analyzed the relationship between native and exotic species richness and abundance on 61 Mima mounds in a prairie underlain by two types of soil geologic substrate (glacial alluvium and basalt) where half of each had been grazed recently. We found that exotic species richness and cover were lowest and native species cover was highest on mounds underlain by basalt bedrock that had experienced recent grazing; on sites underlain by glacial alluvium there was little difference between more and less recently grazed mounds. Moreover, we found that exotic abundance was greatest at the top of Mima mounds compared to the sides; the opposite was true for native species. Our findings indicate that geologic substrate and grazing history may interact to determine plant distribution in semi-arid prairie communities. This study demonstrates that land managers aiming to restore native plant communities need to consider the synergistic effects of multiple environmental factors in order to focus restoration funding and effort.