Population monitoring of game species is essential for supporting regulated hunting programs and for guiding effective conservation of these species in response to climate change, altered land use, and other potential stressors. However, indirect measures of population trend are the best available monitoring data in many situations. We used a 20-year timeseries of indices computed from three independent sources of data (e.g., hunter encounter surveys, harvest records, and citizen science observations) as proxies for the mule deer (Odocoileus hemionus) population status within a 26,400 km2 region of northeastern California. Results suggest that the deer population has steadily increased at an average rate of 1–2% annually, with adult male deer increasing at a faster rate. Periodic oscillations in population were strongly associated with winter severity. Populations appear to respond negatively in the two years following years of heavy snowfall, and the effects of climate change on snowfall may explain the overall increasing population trend. The stability of these apparent patterns is uncertain, and it is unclear whether the trend could be an ecological trap. Efforts underway to apply more robust monitoring methods (e.g., models using data from fecal DNA and aerial surveys) are expected to help answer these questions.