This study investigates the development of a novel, eco-friendly composite material utilizing coconut fibers as reinforcement. The composite prioritizes a balance of affordability, weight reduction, and achieving exceptional mechanical properties, including high strength and hardness. To achieve cost-effectiveness, the isophthalic polyester resin was selected as the matrix material. To enhance the interfacial adhesion between the coir fiber and the polymer matrix, an alkaline treatment was employed. Furthermore, activated carbon was incorporated as a filler to potentially influence the composite’s properties. The mechanical performance of the composites was evaluated through tensile strength and flexural strength. Four distinct composite formulations were investigated: one without activated carbon (0 wt%) and three with varying activated carbon content (5 wt%, 10 wt%, and 15 wt%). The results revealed that composites without activated carbon show better results than the composites with it in both terms of flexural strength and tensile strength. There was a decrement of 39.74% in tensile strength when 15% activated carbon was used, whereas there was a decrement of 62.19% in the case of flexural strength at the same composition of 15% activated carbon. Hence, the experimental investigation reveals that these components are not ideal with each other to produce fiber composites with high mechanical properties.