As for other taxonomic groups, and relative to their land area, islands support a disproportionately large share of the world’s mammal species, but also a disproportionate share of mammal extinctions and threatened mammal species. The extinction-proneness of island species is exemplified by the loss of two Australian island-endemic mammal species in the last decade. Island species tend to be susceptible to novel threats because of their typically small population sizes, limited genetic variation, predator naivety and life history characteristics. Somewhat conversely, islands have also provided conservation security for some mammal species that have disappeared from their formerly extensive mainland ranges due to threats introduced to mainland areas but not yet to islands. We review the fate of the world’s island mammal species over the last few centuries relative to that of the mammal fauna of mainland areas, and describe the factors that have most influenced that fate. We quantify and summarise the current conservation state of the world’s island-endemic mammals. We conclude that the dismal rate of extinctions of island-endemic mammal species is likely to continue into the future unless there is more concerted attempt to enhance island biosecurity, spread extinction-risk, eradicate priority pests from priority islands, and engage better with island communities.