C4 grasslands are a major global ecosystem with an important role as the primary source of food resources and agricultural land for the planet. Despite this, the causes and timeline of their expansion are still not fully known and appear to be variable in different parts of the world. By combining phytolith and stable isotope methods, we produce robust estimates of late Miocene C4 vegetation composition and compile a regional vegetation record through time from the late Miocene through present in southwestern Montana (USA). These estimates indicate the fairly rapid rise of C4 grasses to peak levels during the late Miocene (Hemphillian) and subsequent decline to moderate levels from the Pliocene through Present. This temporal pattern indicates significant interplay between climatic and tectonic drivers, with the late Miocene rise of C4 grasses triggered by regional aridification, and the return to lower (modern) abundances due to ecosystem restrictions linked to recent regional uplift driven by mantle buoyancy associated with the Yellowstone hotspot.