This study investigates the potential of using biochar derived from waste cashew nut shells (Anacardium occidentale) alongside coir fiber (Cocos nucifera) to enhance the mechanical properties of vinyl ester-based composites. By employing a slow pyrolysis technique, waste cashew nut shells were converted into biochar, which was subsequently incorporated into the composite matrix at varying weight percentages of 5, 10, and 15%. The results indicated a significant enhancement in the flexural strength, tensile strength, and water absorption characteristics of the composites compared to the neat vinyl ester matrix. The use of Anacardium occidentale is cost-effective due to its abundant availability as agricultural waste. These findings highlight the effectiveness of biochar as a reinforcing agent while demonstrating the viability of using agricultural waste as a sustainable resource. This research not only contributes to the understanding of biochar’s diverse applications but also emphasizes the importance of biomass waste valorization, promoting both energy efficiency and environmental sustainability in the development of high-performance composite materials.