Delaying flame propagation in the event of a fire can increase the likelihood of preserving life and alleviating property damage. Here, a strategy for flame retardant polymer composite foam is proposed, which enables the improved performance, good formability, and reduced environmental burden while burning. The strategy is to incorporate sylilated nanocellulose into a polyurethane matrix containing a conventional flame retardant, Tris(2-chloroethyl) phosphate (TCEP). This strategy leads to the generation of char layer faster during combustion, resulting in a delayed flame propagation. The limiting oxygen index (LOI) of the samples increased by 28%, and the production rate of toxic gas emission was considerably reduced. The chemical, thermal, mechanical, and morphological analyses were carried out to understand the underlying physics.