Understanding the core distributions of globally-threatened species is important for conservation, especially in areas where habitat loss and fragmentation are high. Ecological niche models are frequently used to estimate areas of occurrence and drive future efforts for conservation, but they are rarely compared to species whose ranges are extremely well known. In the East Usambara Mountains, Tanzania, the distribution of the Endangered Long-billed Forest-warbler Artisornis moreaui has been characterised by repeated surveys from 2005 to 2017 along 57 unique 2 km long transects, each with 10 fixed points 200 m apart, complemented by georeferenced opportunistic observations of this species from 2000 to 2017. Precise estimates of the geographical distribution and occupied area were obtained from the spatial distribution using these detections and compared to ecological niche models created based on these points. We compared how well models predicted the distribution of A. moreaui and the specific habitats it requires and what predictor variables are most important for ascertaining its distribution. We found that A. moreaui has a much smaller distribution than predicted by our models or as reported by international conservation organisations, and that the actual distribution is fragmented into about eight main subpopulations that are mostly (77%) in the protected Amani and Nilo Nature Forest Reserves. Conservation actions should focus on these core areas of distribution, peripheral areas of distribution outside the formally protected areas, and, where possible, restoration of corridors linking these fragmented populations to ensure the future of this species.