Biobased benzoxazine composites were designed and prepared using renewable bio-phenol (cardanol), newly synthesized 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)-urea diamine and eco-friendly borassus aethiopum flower carbon (ABAFC) reinforcement through appropriate experimental conditions. The molecular structure of the DOPO-urea diamine and DOPO-urea diamine-based cardanol-benzoxazine monomers was confirmed using spectral analysis. The biobased benzoxazine matrix and varying weight percentages (1, 3 and 5wt.%) of the bio-carbon-reinforced composites were characterized by different analytical techniques. Result obtained from different studies showed that the glass transition temperature, percentage char yield and contact angle values were increased with the increase in weight percentage of bio-carbon reinforcement. Further, it was also ascertained that the results obtained from corrosion studies using hybrid bio-carbon-reinforced cardanol-benzoxazine composites as the coating materials for steel specimen act as efficient protecting materials. Thus, the overall results suggested that the DOPO urea diamine-based cardanol-benzoxazine composites reinforced with bio-carbon can be used for high-performance corrosion-resistant coating applications.