Free-living species vary substantially in the extent of their spatial distributions. However, distributions of parasitic species have not been comprehensively compared in this context. We investigated which factors most influence the geographic extent of mammal parasites. Using the Global Mammal Parasite Database we analyzed 17,818 individual geospatial records on 1,806 parasite species (encompassing viruses, bacteria, protozoa, arthropods, and helminths) that infect 396 carnivore, ungulate, and primate host species. As a measure of the geographic extent of each parasite species we quantified the number and area of world ecoregions occupied by each. To evaluate the importance of variables influencing the summed area of ecoregions occupied by a parasite species, we used Bayesian network analysis of a subset (n=866) of the parasites in our database that had at least two host species and complete information on parasite traits. We found that parasites that covered more geographic area had a greater number of host species, higher average phylogenetic relatedness between host species, and more sampling effort. Host and parasite taxonomic groups had weak and indirect effects on parasite ecoregion area; parasite transmission mode had virtually no effect. Mechanistically, a greater number of host species likely increases both the collective abundance and habitat breadth of hosts, providing more opportunities for a parasite to have an expansive range. Furthermore, even though mammals are one of the best studied animal classes, the ecoregion area occupied by their parasites is strongly sensitive to sampling effort, implying mammal parasites are undersampled. Overall, our results support that parasite geographic extent is largely controlled by host characteristics, many of which are subsumed within host taxonomic identity.