Nowadays, multivalent batteries are not widely commercialized in contrast with monovalent batteries because their development faces more obstacles, which partly derive from the typically higher charge density of multivalent ions. This property results in poor capacity retention and reversibility, and low coulombic efficiency since the insertion of multivalent ions strongly destabilizes the host structure. To overcome these issues, a close-to-amorphous cathode material has been prepared containing cations (Ca) larger than those to be inserted (Mg). Specifically, low-crystallinity CaMn₂O₄ (CMO) has been prepared and tested as a cathode for aqueous magnesium and calcium batteries. Employing XPS and ICP-OES, calcium and magnesium ions are shown to be inserted-extracted into-from the cathode, the performance being better for magnesium insertion because of its higher diffusion coefficient. In situ Raman analysis reveals that CMO evolves into a birnessite-type structure during the initial electrochemical cycles in both aqueous magnesium and calcium media. The gravimetric capacity obtained for cycle 50 at 263 mA g^-1 in aqueous 1.0 M Mg(NO₃)₂ has a value of 93 mAh g^-1, while for aqueous 1.0 M Ca(NO₃)₂, it is only of 46 mAh g^-1. The results obtained for the magnesium electrolyte compare well with other cathode materials based on manganese oxides.