Nanorods based on ε-MnO2 were synthesized by simple but aggressive acid leaching of spent Zn-Mn battery cathode following a chemical pathway involving the partial cathode dissolution to form Mn2+ and Zn2+, the oxidation of Mn2+ to Mn3+ in the leach liquor, and the crystallization of Mn3+ in MnO2 nanorods in the presence of graphite. Its activity, close to a commercial reference of γ-MnO2, was mainly associated with reducibility, the amount of surface and lattice oxygen, and the average oxidation state. Mn2O3-based nanorods were also synthesized by means of acid leaching but in the absence of graphite and ZnO, giving rise to a material with considerably lower activity. The differences in the formation of Mn-phases were related to a limited ionic oxidation step due to the lack of presence of graphite and ZnO. These findings contribute to changing the current paradigm on useful and profitable toxic waste to obtain high-value products by simple chemical routes.