Bacterial contamination of surfaces and the associated infection risk is a significant threat to human health. Some natural antibacterial polymers with low toxicity are promising coating materials for alleviating pathogenic colonization on surfaces. However, widespread application of these polymers as antibacterial coatings is constrained by coating techniques which are not easily scalable due to stringent reaction conditions. Herein, thiol-ol reaction involving oxidative conjugation between thiol and hydroxyl groups is demonstrated as a facile technique to graft two natural polymer derivatives, agarose (AG) and quaternized chitosan (QCS), as antibacterial coatings on polymer and metal substrates. The substrate surfaces are first treated with oxygen plasma followed by UV-induced grafting of the polymers under atmospheric conditions. Dimercaprol, a FDA-approved drug, is used as both surface anchor and cross-linker of the polymer chains during grafting. The AG coating achieves >2 log reduction in Pseudomonas aeruginosa and Staphylococcus aureus biofilm formation, while the QCS coating reduces bacterial count from contaminated droplets on its surface by >95%. The coatings are noncytotoxic and exhibits a high degree of stability under conditions expected in their potential applications as antibacterial coating for biomedical devices (for AG), and for preventing pathogen transmission in the environment (for QCS).