Understanding why species at risk select certain habitat and what components of their life history influence changes in numbers can help mitigate population declines. The Torngat Mountains caribou herd in northern Quebec-Labrador, Canada, is declining, and few studies have examined the potential causes of this decline. We fitted 9 Argos and 26 global positioning system (GPS)-collars on 35 adult caribou (25 female, 10 male) between 2011 and 2016 to assess seasonal habitat selection at 2 spatial scales, current and future population trends, and interactions with the neighboring Riviere-George migratory caribou herd. The caribou of the Torngat Mountains herd selected areas with abundant food resources in winter and where prevalence of insects was lower in summer. They did not avoid areas where predation risk was high during calving. Spatial overlap with the Riviere-George herd range decreased from 1990 to 2015 and was correlated with the size of the Riviere-George population, which declined drastically during this period. The decline of the Torngat Mountains population was principally attributed to the low survival of adult females (0.72 annual survival rate) owing to subsistence harvest (9/24) and predation (7/24). Demographic models revealed that the growth rate of the population (l) could vary from 0.83 (current) to 0.94 following a decrease in harvest pressure. Using demographic scenarios, we showed that the Torngat Mountains herd could continue to decrease if no management actions were taken to increase adult female survival.