Hunter-collected data and samples are used as indices of population performance, and monitoring programs often take advantage of such data as ecological indicators. Here, we establish the relationships between measures of skeleton size (lower jawbone length and hind-leg length) and autumn carcass mass of slaughtered individuals of known age and sex of the high Arctic endemic Svalbard reindeer (Rangifer tarandus platyrhynchus). We assess these relationships using a long-term monitoring dataset derived from samples of hunted or culled reindeer. The two skeleton measures were generally strongly correlated within age class. Both jaw length (R2 = 0.78) and hind-leg length (R2 = 0.74) represented good proxies of carcass mass, but these relationships were mainly driven by an age effect (i.e., due to growth). The skeleton measures reached an asymptote at 4–6 years age. Accordingly, strong positive correlations between skeleton measures and carcass mass were mainly evident in young age classes (range r [0.45–0.84] for calves and yearlings), while in adult age classes, these relationships weakened due to skeletal growth ceasing in mature animals as well as the strong impacts of annual environmental fluctuations, causing increased variance in mass with age. Although body mass is often viewed as the ‘gold standard’ in ungulate monitoring, our results indicate that skeleton measures collected by hunters may provide similar valuable information about variation in ecological processes, particularly for the younger age classes. Such harvest-based monitoring proxies may improve the management’s access to indicators of changing environmental conditions.