Summary C₄ photosynthesis is a key innovation in land plant evolution, but its immediate effects on population demography are unclear. We explore the early impact of the C₄ trait on the trajectories of C₄ and non‐C₄ populations of the grass Alloteropsis semialata. We combine niche models projected into paleoclimate layers for the last 5 million years with demographic models based on genomic data. The initial split between C₄ and non‐C₄ populations was followed by a larger expansion of the ancestral C₄ population, and further diversification led to the unparalleled expansion of descendant C₄ populations. Overall, C₄ populations spread over three continents and achieved the highest population growth, in agreement with a broader climatic niche that rendered a large potential range over time. The C₄ populations that remained in the region of origin, however, experienced lower population growth, rather consistent with local geographic constraints. Moreover, the posterior transfer of some C₄‐related characters to non‐C₄ counterparts might have facilitated the recent expansion of non‐C₄ populations in the region of origin. Altogether, our findings support that C₄ photosynthesis provided an immediate demographic advantage to A. semialata populations, but its effect might be masked by geographic contingencies.