This study introduces woodnut fibre as a new natural fibre reinforcement for composite materials. This study involved the fabrication of woodnut fibre-reinforced polyester composites with four different fibre loading, both with and without ZnO doping, to evaluate how fibre loading affects the mechanical properties of the composites. The results indicate that adding 1 wt.% of total composite weight to the 20% fibre polyester composites significantly enhances the tensile strength by 17.54% (from 22.4 to 26.45 MPa). Peak flexural strength occurred at 25% loading, further increasing by ~222.65% with ZnO doping. The doping enhanced the impact strength by 127.07% at a 15 wt.% of fibre loading. With 1% ZnO doping, the maximum hardness increases to 455.5 HL at 15% fibre loading. TGA revealed that the composite without ZnO doping exhibited the highest degradation rate of 0.85%/°C. The findings indicate that woodnut fibre—particularly when modified with ZnO nanoparticles – offers significant promise as a sustainable and high-performance reinforcement for composite materials which are now used in the building industry for decorative laminates, wall panels, ceiling boards and insulation. Also used in consumer goods including helmets, luggage shells, electronic casings, storage boxes, packaging items like moulded trays, cushioning inserts and biodegradable containers.