The rapid emergence of multidrug-resistant (MDR) bacteria represents a critical global health threat, underscoring the urgent need for alternative antimicrobial strategies beyond conventional antibiotics. In this study, we report the synthesis of novel biobased antimicrobial polymers bearing quaternary ammonium salts, derived from sustainable feedstocks, maleic anhydride, dimethylaminobenzaldehyde, and furfurylamine. The functional tricyclic oxanorbornene lactam monomer is polymerized via ring opening metathesis polymerization, yielding well-defined polymers with controlled molar masses and low dispersity. Structural characterization is performed using 1D and 2D nuclear magnetic resonance (NMR) spectroscopy, and the polymerization kinetics is monitored by online 1H NMR spectroscopy. The quaternized biobased polymers demonstrate potent broad-spectrum antimicrobial activity against three clinically isolated MDR bacterial strains. They exhibit minimum inhibitory concentrations (MICs) that are significantly lower than those of several conventional antibiotics while also showing low hemolytic activity toward mammalian cells. This study highlights the potential of bioderived ROMP polymers as promising, sustainable antimicrobial polymers for combating the growing threat of antimicrobial resistance.