Three structurally varying benzoxazine monomers (Ml-a, Ml-i, and Ml-d) were synthesized using magnolol (Ml) against aniline (a), imidazole propylamine (i), and dimethylaminopropylamine (d), and the terminal allyl groups of the benzoxazines were epoxidized. Fourier transform infrared spectroscopy, 1H-nuclear magnetic resonance spectroscopy, and high-resolution mass spectroscopy data revealed the successful formation of the monomers. Among all benzoxazine monomers, Ml-a exhibited the lowest polymerization temperature of 220 °C, whereas the benzoxazine polymerization temperature reduced to 191 °C in Ml-a-ep along with an additional polymerization peak at 135 °C, which corresponds to epoxy ring opening. Thermogravimetric analysis revealed that poly(Ml-a-ep) possessed the highest thermal stability of 48% residual mass at 850 °C with limiting oxygen index and heat resistance index values of 37 and 203, respectively. Ml-d-ep possessed the highest antimicrobial activity with a maximum inhibition zone of 41 mm and 32 mm against Escherichia coli and Staphylococcus aureus bacteria. The 2,2-diphenyl-1-picrylhydrazyl antioxidant assay disclosed that Ml-d among benzoxazines and Ml-d-ep among epoxidized benzoxazines possess the highest radical scavenging activity at a concentration of 100 μg/mL. The anticorrosion and hydrophobic studies revealed that poly(Ml-d) possess the highest corrosion inhibition efficiency of 99.9% and a water contact angle value of 149 °. The computational data obtained from density functional theory support the highest corrosion prevention ability of poly(Ml-d). The monomers confirmed the anticancer activity against breast cancer MCF-7 (Michigan Cancer Foundation-7) cell lines through computational molecular docking studies. The toxic profile of the monomers in the current study from ADMET (absorption, distribution, metabolism, excretion, and toxicity) and MTT assay reveals low biocompatibility.