Agricultural waste by-products such as cashew nut shell liquid and rice husk ash were utilized for the syntheses of card-bisphenol (C) and precipitated bio-silica (S), respectively. A hydroxyl terminated card-bisphenol based benzoxazine resin (CHB) was synthesized and then converted it into soft-foam namely benzoxazine-co-urethane pre-polymer (CHB-UF) by polymerizing with hexamethylene diisocyanate (HMDI) in the absence of any catalysts or foaming agents. Subsequent thermal ring opening polymerization of CHB-UF at 200 °C provides poly(benzoxazine-co-urethane) hard-foam (PCHB-UF) with little shrinkage. Similarly, different weight ratios of precipitated bio-silica (0–15 wt%) reinforced bio-based PCHB-UF hard-foam composites (PCHB-UFS) were prepared to utilize them for high-k dielectrics and hydrophobic applications. The foam microstructure was ascertained through the field emission scanning electron microscopy (FE-SEM). The reinforcement of bio-silica into the CHB-UF foam increases the value of dielectric constant according to the increase in weight percentage concentration, which might be due to the reduction in volume of voids. The neat PCHB-UF possesses the value of dielectric constant of 7.04 with lower value of dielectric loss of 0.01. Similarly, the values of dielectric constant and dielectric loss of 15 wt% bio-silica reinforced composites (PCHB-UFS15) are 10.1 and 0.08 respectively. The PCHB-UFS15 has shown hydrophobic nature with the value of water contact angle of 135°. Data obtained from different studies suggest that the bio-foam composites obtained from sustainable sources can be considered as an efficient dielectric materials used for electronics insulation