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Modern savannah grasslands were established during the late Miocene and Pliocene (8-3 million years ago)(1,2). In the tropics, grasslands are dominated by grasses that use the C-4 photosynthetic pathway, rather than the C-3 pathway. The C-4 pathway is better adapted to warm, dry and low-CO2 conditions, leading to suggestions that declining atmospheric CO2 levels(3), increasing aridity and enhanced rainfall seasonality allowed grasses using this pathway to expand(1,2,4) during this interval. The role of fire in C-4 expansion may have been underestimated(1,5-7). Here we use analyses of pollen, microscopic charcoal and the stable isotopic composition of plant waxes from a marine sediment core off the coast of Namibia to reconstruct the relative timing of changes in plant composition and fire activity for the late Miocene and Pliocene. We find that in southwestern Africa, the expansion of C-4 grasses occurred alongside increasing aridity and enhanced fire activity. During further aridification in the Pliocene, the proportion of C-4 grasses in the grasslands increased, while the grassland contracted and deserts and semi-deserts expanded. Our results are consistent with the hypothesis(1,5-7) that ecological disturbance by fire was an essential feedback mechanism leading to the establishment of C-4 grasslands in the Miocene and Pliocene.