The present study reports the design and development of allyl trisphenol (Ah)-based benzoxazines for high thermal stability, corrosion-protective and antimicrobial coating applications. The monomers were synthesized and structurally characterized using ATR-FTIR, 1H-NMR and HRMS analyses. DSC analysis revealed a low temperature curing for Ah-ff (192°C), while TGA measurements showed excellent thermal stability with T max values of 479°C for poly (Ah-ff) and 481°C for poly (Ah-eha), accompanied by high char yields (37% and 34%) and LOI values up to 32, indicating the possibility of flame retardant applications. Reinforcing poly (Ah-chm) with 10 wt% cashew nut shell carbon (CNSC) further enhanced its properties, achieving T max value of 455°C, 37% char yield and LOI of 34. SEM analysis of CNSC reinforced composites showed dense filler dispersion and excellent interfacial bonding. Hydrophobicity tests revealed the water contact angle of 133° ± 2° for poly (Ah-eha), while the poly (Ah-chm)+10% CNSC composite exhibited near superhydrophobic behavior (150° ± 2°). Corrosion resistant studies on mild steel (MS) demonstrated 98%–99% corrosion inhibition efficiency for all polybenzoxazines coated over MS specimen. Which was further validated by density functional theory (DFT) studies. Antimicrobial tests showed inhibition zones of 14 mm (E. coli) and 9 mm ( S. aureus ) for Ah-an. DFT analysis supported the experimental findings, correlating electronic structure with curing reactivity and surface adsorption. Collectively, these results highlight the potential of Ah-based benzoxazines and their CNSC composites for next-generation protective coatings, structural adhesives, and high-performance composites.