<p>Fiber-reinforced epoxy composites are extensively used in aerospace, automotive, and sports industries due to their exceptional properties. Ensuring quality and production efficiency for composite manufacturing requires optimal curing and processing conditions without compromising the structural integrity. Traditionally, techniques such as differential scanning calorimetry (DSC) and dynamic mechanical analysis are used to optimize curing conditions. Still, these methods are confined to the laboratory and may not accurately reflect curing behavior during composite production. Herein, we have used a thin, flexible, and cost-effective screen-printed cellulose paper sensor for real-time cure monitoring of flax fiber-reinforced bio-epoxy composites. The sensor-enabled online monitoring of the composite curing behavior during the vacuum infusion process was carried out at various temperatures. The sensor demonstrated excellent reproducibility and durability under different temperature conditions, and the online monitoring results showed a good correlation with the DSC cure kinetic model. Furthermore, the fibrous structure of the paper-based sensor contributes to the mechanical integrity of the composite, offering reinforcement benefits. The sensor’s facile fabrication, low cost, and reinforcement characteristics present significant potential for improving process control and enhancing the production efficiency of composite structures while ensuring stringent quality control.</p> Graphical abstract <p></p>