Although Li2MnO3 exhibits high capacity via anionic oxygen redox, it suffers from rapid capacity decay owing to structural disordering accompanying irreversible Mn migration and O2 release. To promote the reversibility of the anionic redox reaction, Li1.8Mg0.3Mn0.9O3 as a novel cathode material, prepared by partially substituting Li+ and Mn4+ of Li2MnO3 with the redox‐inactive Mg2+ as a structural stabilizer is proposed. Li1.8Mg0.3Mn0.9O3 delivers a high specific capacity and energy density of ≈310 mAh g−1 and ≈915 Wh kg−1, respectively. In particular, the power‐capability and cycle performance of Li1.8Mg0.3Mn0.9O3 greatly surpass those of Li2MnO3. Through first‐principles calculations and various experiments, it is revealed that Mg substitution effectively suppresses the Mn migration by stabilizing Mn cations in the original sites at the charged state. The energetically stabilized layered structure disfavors the distortion of the MnO6 octahedra, which induces the oxygen dimer (OO) formation through the metal–oxygen decoordination, thus mitigating oxygen release.