<p>The rise in demand for lightweight, eco-friendly, and cost-effective structural materials has shifted researchers’ focus from synthetic fiber/filler-reinforced polymer composites to sustainable polymer composites. In this context, the reinforcing materials obtained from agricultural waste have emerged as one of the potential alternatives to the synthetic fillers, as these materials not only reduce the dependence on virgin raw material but also provide a solution to the waste management problem. This review critically examines the work done so far on agro-waste utilization in polymer composites. The article highlights the role of these fibers or fillers in enhancing material properties under mechanical and tribological load conditions. The agricultural waste reinforcement consists of fibers, particles, powder, and other residues, which helps in significant improvement in material properties. The applications and challenges related to the agro-waste composites are also discussed. Findings indicate that filler strength, modulus, fiber orientation and length, reinforcement and matrix interfacial adhesion, and filler content significantly impact the physical, mechanical, and tribological properties of polymer composites. The article also addresses the importance of agro-waste materials as next-generation reinforcements for applications in the automotive, packaging, construction, and electronics industries. Thus, the need for performing a focused study on waste-derived fillers rather than fibre alone is justified.</p>