I monitored the mammals of the Boston Harbor Islands through various means from 2010 to 2016 in order to create a baseline inventory of species and assess the possibility of additional, focused, research. I employed camera trapping, visitor sightings, and animal-sign surveys to assess the presence of medium- and large-sized mammals. Small mammals were more closely monitored on 2 of the islands—Bumpkin and Peddocks— using short-interval mark–recapture trapping. Small mammals appear to be permanent residents on the islands and undergo significant population fluctuations, which is unusual for insular populations. Medium and large species likely move between islands and the mainland. Larger species, i.e. Canis latrans (Coyote) and Odocoileus virginianus (White-tailed Deer), have been observed with offspring on some of the islands, while medium-sized mammals sighted were single adults. These islands offer a unique situation in which to study insular populations of mammals because the presence of larger animals varies while smaller species cannot move freely between islands or to the mainland.