Manganese-based materials have unique redox properties, attractive to many applications in the energy, environment and biomedical sectors. In particular, Mn3O4 is known to act as both an oxidising and a reducing agent. However, pH-dependence of the redox properties of Mn3O4 nanoparticles is not yet well understood. In this study, Mn3O4 nanoparticles were mechanochemically synthesised and their redox catalytic activities were studied under different pH conditions. Mn3O4 nanoparticles with sizes between 10 and 90 nm were prepared via a mechanically-induced solid-state displacement reaction between MnCl2 and K2CO3 and subsequent heat treatment. The effect of pH on the oxidative decomposition of a model organic dye revealed that Mn3O4 nanoparticles showed oxidative catalytic activity only when pH was below 5. At a higher pH, Mn3O4 nanoparticles showed strong free-radical scavenging activities, suitable for reducing oxidative stress in many locations of the body and for creating new design of energy-storage materials.