The synthesis of porous activated carbon (specific surface area=1883?m² g^?1), Fe₃O₄ nanoparticles, and carbon–Fe₃O₄ (C?Fe₃O₄) nanocomposites from local waste thermocol sheets and rusted iron wires is demonstrated herein. The resulting carbon, Fe3O4 nanoparticles, and C?Fe₃O₄ composites are used as electrode materials for supercapacitor applications. In particular, C?Fe₃O₄ composite electrodes exhibit a high specific capacitance of 1375?F?g^?1 at 1?A?g^?1 and longer cyclic stability with 98?% capacitance retention over 10?000?cycles. Subsequently, an asymmetric supercapacitor, namely, C?Fe₃O₄∥Ni(OH)₂/carbon nanotube device, exhibits a high energy density of 91.1?Wh?kg^?1 and a remarkable cyclic stability, with 98?% capacitance retention over 10?000?cycles. Thus, this work has important implications not only for the fabrication of low‐cost electrodes for high‐performance supercapacitors, but also for the recycling of waste thermocol sheets and rusted iron wires for value‐added reuse.