Sustainable poly(geo‐benzoxazine) composites have been developed from the combination of inorganic reinforcement (aluminosilicate from fly ash) and organic resin (benzoxazine) for high‐k dielectrics. A bio‐sourced card‐bisphenol‐based water‐soluble disodium salt of benzoxazine (BBC‐aipa‐Bz) was synthesized using 5‐aminoisophthalic acid and paraformaldehyde through Mannich condensation. The BBC‐aipa‐Bz was mixed with different weight percentages of fly ash (FA) and sodium silicate; then the mixture was pelletized and cured at 200°C to obtain poly(geo‐benzoxazine) composites (BBC‐aipa‐PBz‐FA5‐50) in the form of a compact thin film. The surface morphology and distribution of the FA‐silica network in the polymer matrix were ascertained from the SEM images. The cure temperature, dielectric behavior, thermal stability, and hydrophobic properties of the prepared composites were studied, and the results infer that the values of dielectric constant and hydrophobicity of the composites increase with increasing weight percentages of FA. The neat BBC‐aipa‐PBz inherently possesses the high dielectric constant value of 6.35 at 1 MHz, and it increased to 12.7 and 23.48 for the 30 wt% and 50 wt% FA reinforcement, respectively. The present study provides a pioneering approach for the development of high‐k dielectrics through the formation of an FA‐based geopolymer network as an integral part of the polybenzoxazine matrix.