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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. [ABSTRACT FROM AUTHOR]