The growing demand for sustainable, high-performance materials has driven the development of natural fiber-reinforced polymers incorporating waste-derived fillers. This study explores the enhancement of sisal fiber-reinforced polyester composites using locally sourced bagasse ash (BA) and granite particles (GP) as single and hybrid fillers. Composites with a fixed 30% sisal content and varying filler loadings (4%, 8%, 12%), including hybrid mixtures (8% total), were fabricated via hand layup and systematically evaluated for mechanical performance and water absorption. This work presents a novel hybrid filler strategy using bagasse ash and granite waste in sisal–polyester composites. Results unequivocally demonstrated that adding these waste-derived fillers significantly improved both the mechanical performance and water absorption resistance compared to the unfilled composite. Specifically, composites reinforced with 8% GP achieved superior physical and static mechanical properties, showing optimal water absorption of 4.2% (vs. 8.12%), a maximum flexural strength of 51.57 MPa (vs. 34.63 MPa), and a compressive strength of 66 MPa (vs. 46 MPa). Conversely, the bio-filler proved optimal for toughness: 4% BA composites displayed the highest impact strength of 42 kJ/m², representing a 110% increase over the unfilled sample, while 8% BA yielded the maximum tensile strength of 40.5 MPa, representing a 62% improvement. These substantial enhancements are primarily attributed to improved interfacial adhesion and effective stress transfer among the matrix, fibers, and fillers, successfully mitigating failure initiation and boosting the overall composite performance. Owing to their improved mechanical properties and reduced water absorption, the developed composites show strong potential for use in wall-partition panels, insulation boards, and interior automotive components. This study demonstrates how agro-industrial waste fillers can be strategically selected or hybridized to tailor the mechanical and physical performance of natural fiber composites. Graphical abstract