In accordance with some embodiments of the present invention, a composite material is prepared by blending a flame retardant modified cellulosic nanomaterial (FR-CN) filler into a polymer, wherein the FR-CN filler comprises a cellulosic nanomaterial (e.g., cellulose nanocrystals (CNCs) and/or cellulose nanofibrils (CNFs)) having a surface functionalized to incorporate a phosphorus-containing moiety. In some embodiments, the FR-CN filler is prepared by reacting hydroxyl groups on the surface of the cellulosic nanomaterial and a halogenated phosphorous-containing monomer (e.g., diphenyl phosphoryl chloride). In some embodiments, the surface of the cellulosic nanomaterial is further functionalized to incorporate an orthogonal functionality selected to enhance the compatibility of the FR-CN filler with the polymer by reacting hydroxyl groups on the surface of the cellulosic nanomaterial and a monomer (e.g., epichlorohydrin when the polymer is an epoxy-based polymer).