Snowpacks in mountain regions with Mediterranean climates are exceptionally sensitive to climate warming. However, these marginal snowpacks are sparsely monitored, limiting our understanding of recent snow losses and constraining our ability to anticipate and manage future changes in mountain water supply. Here we present snowMapper v1.0, a modular, physics-informed, machine-learning-based model for reconstructing daily snow cover at high spatial resolution using satellite imagery and gridded climate products. snowMapper is fully configurable and features dedicated modules for masking, preprocessing, snow binarization, snow reconstruction, spatiotemporal aggregation, and validation. It performs with exceptionally high skill. Using snowMapper, we generate a monthly snow-cover climatology for ten of Greece’s highest mountain massifs for the period 1984–2025. Our results reveal a rapid and widespread decline in snow cover area (SCA), amounting to a ~58 % reduction relative to the 1984–2025 mean. We identify sustained warming throughout the snow season as the primary driver of this decline. Precipitation changes correlate with SCA only in early and mid-winter, underscoring the dual role of air temperature in controlling both accumulation (via snowfall fraction) and ablation processes. The North Atlantic Oscillation exerts only a modest influence on mid-winter SCA, and primarily when acting in conjunction with the Arctic Oscillation, representing a stark contrast to patterns observed in western Mediterranean mountain ranges. Finally, the absence of a strong relationship between SCA and the Atlantic Multidecadal Oscillation reinforces the conclusion that the observed trends lie outside the bounds of natural climate variability.