Domestic dogs (Canis lupus familiaris) and cats (Felis catus) in natural reserves can affect wild mammal populations through predation, diseases, and disturbance, particularly when habitat patches are small. In Estero El Salado, a remnant mangrove in Puerto Vallarta, Mexico, we used camera trapping to obtain independent observations (n = 1224) to determine abundance, spatial overlap, and activity of medium-sized mammals (domestic dog, domestic cat, northern raccoon [Procyon lotor], Virginia opossum [Didelphis virginiana], gray fox [Urocyon cinereoargenteus], nine-banded armadillo [Dasypus novemcinctus], and jaguarundi [Herpailurus yagouaroundi]). Our efforts constitute the first photographic record for the reserve. Relative abundance (capture frequency referred to 100 trap-days) was largest for raccoons (62.11), followed by dogs (13.31) and cats (5.28). Racoons and cats showed large spatial overlap (0.951), as did nine-banded armadillo and foxes (0.973). Dogs and cats showed different activity patterns from all wild species (P < 0.05) but were similar to each other (P = 0.408), with large temporal overlap (82.8%). Wild species also displayed large temporal overlap (i.e., opossums–raccoons, 80.6%). Wild mammals were predominantly nocturnal, whereas dogs were active throughout the 24-h day/night cycle and cats were mainly crepuscular, suggesting that wild species might avoid domestic species by displaying differential activity patterns. We propose control measures within and around the reserve and an investigation of the possible effects of domestic species on wild species through studies of diet and parasites.