Rechargeable magnesium batteries (RMBs) are a promising post-lithium battery technology that benefits from the use of a Mg metal anode, which provides a high volumetric capacity (3833 mAh/cm3), low reduction potential, and dendrite-free deposition. In parallel to the development of novel electrolytes compatible with Mg, the future realisation of rechargeable magnesium batteries demands cathode materials with high-energy densities and suitable Mg intercalation kinetics. In this mini-review, the focus will be laid on the high-voltage intercalation MgMn2O4 oxyspinel cathode (and substituted derivatives). We aim at providing an updated understanding of the reaction mechanisms occurring during (de)magnesiation of MgMn2O4, and the role of Mg/Mn anti-site defects in its electrochemical behaviour. We then critically evaluate the performance of MgMn2O4 in organic and aqueous-based electrolytes, highlighting their merits and challenges, and provide an overview of the most recent developments to improving battery performance. Finally, we will highlight key areas that require further attention to provide an understanding of their charge storage behaviour.