Bio-based benzoxazine (CBPA-Bz) resin was synthesized using card-bisphenol derived from cardanol, and aniline with paraformaldehyde through Mannich condensation followed by thermal polymerization to obtain polybenzoxazine (CBPA-PBz). Free hydroxyl groups of the polybenzoxazine matrix developed on polymerization have been modified in the form of ester via esterification with dodecenyl succinic anhydride (DSA) at high temperature through in situ reaction. The non-activated bio-carbon (Cna) and acid-activated bio-carbon (Caa) were derived from Phragmites karka (p.karka). Different weight percentages of bio-carbons reinforced hybrid CBPA-DSA-PBz-Cna/aa polybenzoxazine composites were prepared, and studied their dielectric, thermal, and water-repellent properties. FTIR and ¹H NMR spectra were used to ascertain the molecular structure of the benzoxazine monomer and polymer composites. The hybrid CBPA-DSA-PBz and its composites exhibit higher values of water contact angle (WCA) between 141o and 165o infer their (super) hydrophobic behaviour compared to its pristine CBPA-PBz matrix (WCA = 140o). The hydrophobic property of the developed polybenzoxazine (CBPA-DSA-PBz) and its composites (CBPA-DSA-PBz-Cna/aa1-10) could provide an authentic dielectric constant and dielectric loss values, because of preventing the effect of moisture on dielectric measurement. The CBPA-DSA-PBz-Cna10 and CBPA-DSA-PBz-Caa10 hybrid composites show high values of dielectric constant, 6.97 and 7.55, respectively.