A new type of trifunctional polybenzoxazine (PTP-a) has been synthesized using trisphenol (TP) and aniline (a) through Mannich condensation. The PTP-a has been subjected to carbonization and doping with SnO2 nanoparticles which yields a metal doped highly porous graphitic carbonaceous conducting material (PTP-a/Sn-C). The prepared PTP-a based tin doped conducting carbon was used as reinforcement/filler for the development of TP-furfuryl amine polybenzoxazine (PTP-ffa) composites with varied weight (1, 3, 5, 10, 15, 20 and 25 wt%) percentages. The increasing weight percentages of the reinforcement (Sn-C) contribute to enhanced thermal stability as well as improved electrical conductivity of the resulting polybenzoxazine composites. It was observed that the maximum degradation (Td) temperature of the PTP-a/Sn-C reinforced PTP-ffa composites reached to 572 °C with 72% char yield. Similarly, the value of electrical conductivity of the developed composites is increased from 3.81 × 10−7S/m to 0.878 S/m and that of dielectric constant increased from 4.29 to 13.32. Data from the electromagnetic interference (EMI) shielding studies infer that the polybenzoxazine composites exhibit excellent EMI shielding efficiency. Among the composites developed, the 25 wt% PTP-a/Sn-C reinforced PTP-ffa composite showed the highest EMI SE of 20.28 dB/mm with 99.99% shielding effectiveness. The present work is considered as cutting-edge concept in the field of metal doped carbonaceous conducting benzoxazine composites for EMI shielding application.