Synthetic textiles are noted for their high performance and ease of production, but many of the polymers used in their manufacture are inherently flammable and require additional treatments. An alternative is to add flame retardants to the polymer melt prior to spinning, but some flame retardants, such as halogenated, have raised concerns about toxicity, environmental impact and bioaccumulation. In this work, a halogen-free flame retardant with maleimide groups was synthesized and added to a polyurethane (PU) solution with pendant furan groups to produce nanofibers by electrospinning. The furan groups of the polyurethane and the maleimide groups of the flame retardant can readily react via the Diels-Alder (DA) reaction. Nanofiber membranes were prepared from PU solutions with different flame retardant contents, and subsequently cross-linked by DA reaction. The resulting nanofiber membranes were characterized before and after the cross-linking process. The cross-linked membranes showed higher solvent resistance and better mechanical properties than the non-cross-linked ones, and the properties were dependent on the flame retardant content. Thermal analysis showed that the flame retardant imparted flame retardant properties to the membranes The results presented in this work demonstrate the versatility of polyurethanes to produce tailor-made polymers, easily processable by electrospinning. The furan groups of the structure contribute to the production of cross-linked nanofibers by DA reaction with a functional cross-linking agent with maleimide groups, in this case a halogen-free flame retardant, which also provides thermal, chemical and mechanical resistance to the nanofibers.