Biomass-based energy storage devices have drawn increasing attention owing to their renewability and sustainability, particularly that the heteroatom-doped carbons derived from natural polymers are regarded as the promising candidates in discovering advanced electrode materials for supercapacitors. This work has developed a facile one-pot fabrication strategy toward synthetic pheomelanin nanoparticles with controllable size and chemical composition (i.e., sulfur content) via the copolymerization of dopamine and cysteine. The resulting synthetic pigment materials possess outstanding thermal stability and are able to directly transform into monodispersed S,N-codoped carbon spheres with unaltered morphology. Compared with conventional polydopamine-based carbon spheres, the present carbonized pheomelanin nanoparticles with electroactive sulfur atoms could possess lower charge-transfer resistance and consequently higher specific capacitance (e.g., 243 F g–1 at 1 A g–1). This research continues to inspire researchers to develop new kinds of energy storage materials based on synthetic biopigment materials.