Susceptibility assessments are crucial in identifying hazards in densely populated mountain areas featuring active internal and external dynamic geological processes. A comprehensive analysis of the interactions among ecological, hydrological, and geotechnical factors was performed to develop a quantitative method for assessing regional landslide susceptibility based on slope failure and landslide formation mechanisms. Using slope geomorphic units (SGUs) that can sufficiently describe the comprehensive features of landforms and hazard-forming factors related to landslides, an integrated model was built to analyze potential slip planes and slope stability by considering the different reinforcement effects of roots on soil, the root anchoring capacities of various vegetation types, and the corresponding vegetation weight loads. Using this SGU-based method, the occurrence possibility of regional-scale landslides was determined by analyzing the propagation directions and maximum moving distances of landslide materials in unstable areas. Taking the landslides in the Dadu River Basin, China, as a case study, a landslide susceptibility map was obtained and validated by conducting a field study and remote sensing interpretation of actual landslides; the assessment results were in accordance with the actual disaster situation. The distributions of zones with high or very high susceptibility (i.e., high-altitude valleys with steep slopes, abundant rainfall, and severe soil erosion) are closely correlated with the slope topography and stability. These findings suggest that the proposed assessment methodology can provide scientific support for preventing or mitigating landslide hazards and may serve as pertinent guidance for regional landslide susceptibility assessments in the Dadu River Basin and beyond.