Costa Rica is one of the most biodiverse countries in the world, hosting almost 4% of all known mammals. Although the local mammals have undoubtedly been more investigated than those of other Central American countries, some regions still appear poorly known to date, such as the Nicoya Peninsula, NW Costa Rica. Our research focuses on the Karen Mogensen Wildlife Refuge, an important component of the Nicoya Peninsula Biological Corridor. This investigation represents the first comprehensive study of mammals of this area, with the aims to provide baseline information on the mammal community and tourism management. The investigation during a 6-year period (2018–2023) revealed the presence of 60 species, belonging to 50 genera, 23 families, and 8 orders, indicating a high species richness of the area. Among these, we highlight the occurrence of the globally Vulnerable Alouatta palliata and Cebus imitator and the Near Threatened Leopardus wiedii and Vampyrum spectrum, along with the recently recognised Lontra annectens, whose status is still Not Evaluated. The influence of tourism on activity patterns of mammals was also investigated. Using camera trapping, 13 species were detected in 537 trap-days and divided based on functional traits. No evidence of impacts caused by the presence of tourists was found on these species. This suggests that the current conservation policies of the Wildlife Refuge appear to be incorporating ecotourism in a wise and thoughtful manner. The information gathered will be a useful tool for the development and implementation of conservation strategies and actions at the local scale in the near future.