The aim of this study was to focus on the development of cardanol‐terminated tetrafunctional benzoxazine (P‐ipda‐C) by reacting with phenolphthalein, isophoronediamine, paraformaldehyde, and cardanol under appropriate experimental conditions. Proton, carbon NMR and FTIR spectroscopic techniques were used to analyze the chemical structure of the synthesized benzoxazine monomer (P‐ipda‐C). The polymerization temperature ( T p ) of P‐ipda‐C (242 °C) monomer was determined by differential scanning calorimetry (DSC). The P‐ipda‐C composites were prepared by integrating different weight percentages (1%, 3%, 5%, 7%, 10%, and 15%) of cardanol‐modified graphitic carbon nitride (g‐C 3 N 4 ) and boron‐doped graphitic carbon nitride (B‐g‐C 3 N 4 ) as reinforcing agents. The T max of the pure poly(P‐ipda‐C) was recorded at 392 °C, while all the developed composites displayed T max values exceeding 425 °C. The theoretical limiting oxygen index (LOI) values calculated for the composites exceeded the critical threshold of 26, indicating their self‐extinguishing properties. The water contact angle (WCA) of poly(P‐ipda‐C) was measured at 137 ± 2°. Notably, polybenzoxazine composites containing 15 wt% of cardanol‐functionalized g‐C 3 N 4 and B‐g‐C 3 N 4 exhibited the highest WCA values of 150° and 151°, respectively, approaching superhydrophobicity. The dielectric constant of the pristine poly(P‐ipda‐C) was measured as 4.29 ± 0.01. In contrast, the P‐ipda‐C polymer composites reinforced with 15 wt% of cardanol‐functionalized g‐C 3 N 4 and B‐g‐C 3 N 4 exhibited the highest dielectric constants of 9.8 and 10.2, respectively, at a frequency of 2 MHz. The results of the thermal, hydrophobic, and dielectric studies demonstrate that P‐ipda‐C polybenzoxazine and its cardanol‐functionalized g‐C 3 N 4 and B‐g‐C 3 N 4 reinforced composites can be suitably exploited for electronic applications.