Continuous population growth and rapid urbanization will outpace the global capacity of solid-waste disposal. Although pyrolysis, in the waste management sector, has been regarded as a savior to bring potential returns, it has struggled against high operation costs. Here, we found that simple pyrolysis of properly mixed wastes [e.g., organic wastes (C) + metal wastes in the form of metal oxides (M O )] can bring about the carbothermic reduction [C(s) + M O (s) → C'(s) + M(s) + CO(g)] caused by the thermodynamic reducibility of metal oxide. This process consequently produced not only nanoporous carbon powder usable for energy storage devices but also nanoporous carbon fully decorated with magnetic nanoparticles useful for magnetic applications. We believe that the carbothermic reduction process, historically used for metal refining, could be a promising alternative to resolve the long-pending issues of the conventional pyrolysis approach as well as to produce useful nanoporous carbon with ease. Our method could be a simple and effective way to transform ubiquitous solid waste into useful resources in the form of nanoporous carbon.