Monogamy is typically considered to have evolved either because biparental care is important for offspring survival, or because males are unable to monopolize more than one female due to females being too dispersed. Here, in the first phylogenetic analysis of the evolution of monogamy in mammals, we show that neither of these explanations is consistent with the distribution of monogamy across mammal species. Monogamy evolved significantly more often in the absence of paternal care than in its presence. Furthermore, monogamy does not normally occur in species where female ranges are large. Rather, the most common feature of mammalian monogamy is that it evolved where females were solitary and occupied small, exclusive ranges, enabling males to monopolize them.