Due to the generation of fat-rich residues from the leather industry, these fat-rich residues are considered a promising feedstock for waste valorization into biofuel applications. Limed flashing waste (LFW), collected from Badr City, Egypt, was subjected to different multi-purpose pretreatment steps, including delimiting, drying, chopping, Soxhlet extraction, and degumming to recover oil. The recovered oil was then transesterified with methanol using commercial CaO as a catalyst. Multi-variable regression modeling was applied to optimize the triglyceride conversion into biodiesel. The optimized reaction conversion yielded a conversion of 97.74% achieved under a methanol-to-oil ratio of 8.93:1, 5.74% w/w catalyst loading, 63.4°C reaction temperature, agitation speed at 300 rpm, and 3 hours of reaction time. GC-MS and FT-IR analysis both confirm the conversion of LFW into biodiesel, highlighting its suitability and feasibility as an alternative source for sustainable energy. It was demonstrated that LFW produced from the leather industry, whose handling and discarding are challenging, can be converted into biofuel. Thereby contributing to waste management and energy demands.