All interactions between multiple species invading together (coinvasion) must be accounted for to predict species coexistence patterns across space. Mutualisms, particularly, are known to influence species’ population dynamics and their invasive ability (e.g. mycorrhizal fungi with partner plants). Yet, while modelling coinvasion, their role in mediating coexistence is overlooked. We build a spatial model of coinvasion of two competing plant species with a shared fungal mutualist to study how mutualism and competition shape the spatial coexistence of competitors. We find that mutualist presence engenders regional (spatial) coexistence between competitors even when local coexistence between them is impossible. Further, increasing mutualist dispersal results in sharp transitions in competitor coexistence outcomes. Finally, differences in mutualist partner dependence and competitive ability interact to produce a variety of local and regional coexistence outcomes. Thus, incorporating mutualism dependence and dispersal behaviour into coinvasion models offers new insights into spatial coexistence and invasive species distributions.