Mountains play a critical role in freshwater supply for downstream populations. Previous research has demonstrated that climate change is impacting mountain streamflow patterns globally, mainly due to changing snow and ice conditions. However, the impact of climate change on mountain groundwater systems has received less attention. In particular, the impact of precipitation phase change (snow to rain) on mountain groundwater systems remains disputed. This study quantifies temporal trends in mountain groundwater levels in Canada and the United States and explores how various climatic, physiographic, and anthropogenic factors affect these trends. Furthermore, we assess how precipitation phase change is affecting temporal patterns in groundwater recharge in mountain regions. We compiled a dataset of 171 mountain observation wells with at least 20 years of continuous monthly data. The Mann-Kendall test for monotonic trend revealed that 54% of these wells have statistically significant temporal trends (p < 0.05) over the period of record, of which 69% were negative, therefore indicating declining groundwater storage. Correlation, regression, and Kruskal-Wallis tests revealed that lower-elevation mountain regions with higher average annual temperatures and lower average annual precipitation are seeing the greatest declines in groundwater storage under climate change. Well locations transitioning from nival to pluvial regimes show increasing winter groundwater levels and decreasing summer levels. Our findings demonstrate that the impact of climate change on mountain water resources extends to the subsurface, with important implications for global water resources.