In Sri Lanka’s hilly terrains underlain by Precambrian meta-sedimentary rocks, tropical weathering processes and unmanaged slope cutting have led to widespread instability. Intense precipitation, combined with physical, chemical, and biological weathering, transforms parent rocks into lateritic soils or crusts with variable strength and permeability. These variations create zones of differing shear strength and erodibility, posing challenges for stable slope design. This study investigates mineralogical changes in residual formations and their parent rocks at a major slope-cutting site along the Sri Lankan Expressway Project. Petrographic analyses, physical and mechanical property tests on rock units, and index property and X-ray diffraction (XRD) tests on residual soils were performed. Results reveal a strong correlation between mineralogical alterations and the reduction in strength parameters of weathered materials, directly influencing slope stability. Findings underscore the need for incorporating mineralogical assessment into geotechnical design for sustainable land use in tropical hilly terrain.