Mn-based spinel-oxide cathode materials are promising for achieving high-energy-density rechargeable Mg batteries (RMBs). However, Mg insertion into them often induces unfavorable phase transformation due to the poor stability of λ-MnO2, leading to capacity fading during cycling. Defect spinel ZnMnO3, which can be regarded as ZnO-stabilized λ-MnO2, is an outstanding exception that allows highly reversible Mg insertion/extraction. To further understand its phase stability, here we investigate wide-range compositions in Mg–Zn–Mn oxide systems and show that the stability of the spinel structure can be significantly improved by compositionally incorporating stable XO (X = Zn, Mg) with λ-MnO2. In particular, (i) the equimolar mixing of XO and MnO2 is critical to obtaining a single-phase cubic spinel structure and (ii) a higher Zn/Mg ratio is effective for preventing the formation of an irreversible rock salt phase to decrease the overpotential during discharge/charge cycling. Consequently, Zn-rich Mg–Zn–Mn oxides with the cubic spinel structure delivered as high as 120 mAh/g discharge capacities repeatedly at an elevated temperature of 150 °C. This work provides a fundamental understanding of the phase stability of Mg–Zn–Mn oxide materials and insights into designing high-performance cathode materials for RMBs.