Snow droughts are increasingly recognized as a critical feature of dry periods in mountain regions, but their impacts on hydrology, ecosystems, and communities remain poorly understood. This knowledge gap hampers adaptation efforts amidst accelerating climate change and declining snow water resources. Using 13 years of data from over 30 Italian headwater catchments, this study reveals that snow droughts profoundly affect mountain socio-ecological systems, with cascading impacts extending downstream. Key findings include increased melt-out events, shortened snow seasons, and significant hydrological changes, even without differences in summer precipitation or temperature. Snow droughts also alter vegetation phenology and productivity and necessitate emergency water management measures, with outcomes influenced by water infrastructure. The study emphasizes the need to view snow droughts as a socio-ecohydrological risk with far-reaching implications for water security in mountain and downstream regions.