A new monomer of bi-functional benzoxazine was synthesised using cardanol (C) and p-phenylenediamine (ppda) under suitable experimental conditions. The curing behaviour of the cardanol-based 1,4-bis(7-pentadecyl-2H-benzo[1,3]oxazin-3(4H)-yl)benzene (C-ppda) benzoxazine monomer was studied by differential scanning calorimetry analysis, and the polymerisation temperature (T p ) of C-ppda benzoxazine was found to be 237°C. Further, the benzoxazine monomer was reinforced with varying weight percentages (5, 10 and 15 wt%) of bio-ash derived from Aerva lanata (AL-ash) to obtain hybrid composites. Thermogravimetric analysis data indicate that AL-ash-reinforced benzoxazine composites possess excellent thermal stability and a flame-retardant behaviour. The morphology of AL-ash- and cardanol-based benzoxazine composites was analysed using field emission scanning electron microscopy (FESEM). The FESEM results indicate homogeneous distribution of AL-ash in the composites. Energy-dispersive X-ray spectroscopy analysis was used to determine the elemental composition of the AL-ash used for the preparation of composites. The value of the water contact angle of poly(C-ppda) was found to be 148°. Data obtained from corrosion studies indicate that the mild steel specimen coated with a benzoxazine matrix and the specimen coated with bio-ash-reinforced benzoxazine composites exhibit an excellent resistance against corrosion. The bio-ash-reinforced composites of cardanol-based benzoxazine can be used in the form of sealants, encapsulants, adhesives, coatings and matrices in microelectronics and automobile applications under high thermal and moist environmental conditions.