Fire hazards remain a major safety concern, with building fires causing severe economic losses and risks to life. Intumescent fire-retardant coatings (IFRCs) provide passive fire protection by forming thermally insulating char layers that shield underlying substrates, yet conventional systems often suffer from toxicity concerns, mechanical instability, and limited durability. This work reports the development of an IFRC reinforced with cellulose nanofibrils (CNFs) derived from corn cob waste. CNFs were produced through alkaline treatment, bleaching, phosphorylation, and TEMPO-mediated oxidation, then incorporated with montmorillonite, sodium hexametaphosphate, and polyurethane at loadings of 0 to 3 wt.% and applied to plywood. Fire performance was evaluated using a laboratory-scale fire test based on a modified ASTM D3806 method, with mass loss, flame spread, and Flame Spread Index (FSI) as key metrics. A CNF loading of 1.5 wt.% delivered the best performance, reducing FSI to 61.68 and mass loss to 19.05%, surpassing both uncoated wood and a commercial fire-retardant coating. Performance gains are attributed to CNF-induced formation of dense, cohesive char layers supported by hydrogen bonding and thermal stabilization. Higher CNF contents led to agglomeration, increased viscosity, and reduced effectiveness. Findings demonstrate that controlled CNF incorporation can significantly improve IFRC performance while providing a viable route for valorizing agro-waste in fire-protective coatings.