Rainfall-induced landslides are among the most recurrent and destructive geohazards in the Himalayan region, where steep slopes, fragile lithology, and intense monsoonal precipitation collectively contribute to frequent slope failures. This study aims to establish rainfall thresholds responsible for slope instability through an integrated physical–numerical modeling framework in the Pithoragarh district of Uttarakhand, India. Five representative slope sites (S1–S5) were selected to capture lithological and geomorphological variability. Laboratory-scale physical modeling was conducted using a rainfall simulator to replicate progressive infiltration and failure behavior under controlled rainfall intensities. Concurrently, numerical simulations were performed using GeoStudio’s SEEP/W and SLOPE/W modules to quantify pore-pressure variation and Factor of Safety (FoS) reduction under saturated conditions. Results revealed a significant decline in FoS from 1.548–2.010 (dry) to 0.748–0.981 (saturated), confirming that rainfall infiltration leads to increased pore-water pressure and loss of matric suction, ultimately triggering slope failure. The integrated results identified a critical rainfall threshold of 45–50 mm cumulative rainfall at an intensity of approximately 28 mm h−1, corresponding to the onset of instability where FoS drops below unity. The close agreement between physical and numerical outcomes validates the robustness of the hybrid modeling approach. The derived intensity–duration (I–D) threshold aligns with global empirical ranges and provides a reliable basis for early warning and hazard mitigation in Himalayan terrain. The study underscores the importance of coupling experimental validation with computational analysis for realistic prediction of rainfall-induced slope failures in data-scarce mountain environments.