Analyses of the distributions of northern Melanesia's avifauna have helped researchers formulate major theories of evolution, speciation, and biogeography. In comparison, the region's mammalian fauna has been the subject of few empirical analyses of biogeography. Knowledge of mammalian biogeography is needed to formulate sound conservation strategies for mammals in the region and will help further advance these theories. We aim to assess the relative importance of biogeographic variables to mammalian zoogeography in northern Melanesia and empirically describe patterns of species richness. Using published literature, museum data bases and our own surveys, we compiled mammal inventories for 75 islands found within northern Melanesia (comprising the Admiralty, Bismarck, D‘Entrecasteaux, Louisiade, Solomon, and Trobriand Archipelagos). We used direct comparisons of family compositions and Ochiai and endemism indices to compare mammal assemblages between island groups and a defined New Guinea source pool region. Generalised linear models and non-parametric multivariate analyses were used to identify, and rank, the importance of local and regional scale abiotic variables for mammalian species richness. Northern Melanesia's islands are depauperate in marsupials and have a moderate diversity of insectivorous bats and a diversity of pteropodid bats that exceeds that known to occur in the adjacent northeastern area of New Guinea. The Solomon Islands are conspicuous for their outstandingly high mammalian endemism. The mammalian fauna of northern Melanesia's islands was related primarily to island size and secondarily to isolation. Island size best explained inter-island variation in total mammal species richness and bat species richness. The linear distance to New Guinea best explained the species richness of non-volant mammals. Our results reveal islands that are in need of survey focus because their mammal inventories are lower than expected for their size, and we highlight areas of endemism where conservation actions might be prioritised to prevent species extinctions.