Manganese oxides represent a class of promising cathode materials for rechargeable aqueous zinc-ion batteries (ZIBs) due to their high energy density, safety, low cost, eco-friendliness and non-toxicity. However, manganese monooxide (MnO) is considered to be inactive for Zn2+ energy storage. Herein, for the first time, we demonstrate the possible application of commercial MnO microsized particles as cathodes for high performance ZIBs. It is interesting to note that electrochemical activation (or oxidation) is observed during the initial charging process, which leads to the formation of porous layered-type MnO2 nanosheets surrounding the MnO surface. As a result, the activated cathode could deliver a maximum specific capacity of 330 mAh g−1 in ZnSO4 aqueous electrolyte at a current density of 0.1 A g−1. In addition, the underlying energy storage mechanism is systematically investigated. The present work provides a new insight into the electrochemical activation strategy for developing advanced cathodes for high-performance zinc-ion batteries.