<p>Natural fibers are increasingly being considered as sustainable alternatives to synthetic fibers for reinforcing polymer composites due to their renewability, low cost, biodegradability, and low environmental impact. Among various natural fibers, flax fiber has attracted significant attention because of its good mechanical properties, availability, and lignin-rich composition. However, most polymeric materials are inherently flammable, making fire safety and thermal stability critical considerations for their use in applications such as construction and automotive components. In this context, the present study investigates the effect of flax fiber reinforcement on the flammability behavior, thermal stability, and mechanical performance of polycarbonate (PC) composites. The composites were prepared using a micro-compounder followed by compression molding with varying flax fiber loadings (0–30 wt%). The novelty of this work lies in exploring the dual role of lignin-rich flax fibers as both mechanical reinforcement and a potential contributor to improved flame resistance in polycarbonate composites. Flammability performance was evaluated using the UL-94 horizontal burning test, where composites containing 30 wt% flax fiber, indicating significant improvement in fire resistance. The flame propagation rate increased initially and reached a maximum at 20 wt% fiber loading, followed by a decrease at higher fiber content, suggesting the influence of char formation and fiber distribution on flame behavior. Thermogravimetric analysis revealed enhanced thermal stability with increasing fiber content, while mechanical testing showed improved strength and stiffness compared to neat polycarbonate. The results demonstrate that flax fiber reinforcement can effectively enhance the fire performance, thermal stability, and mechanical properties of polycarbonate composites, making them promising candidates for sustainable engineering applications.</p> Graphical abstract <p></p>