Structural defects, e.g. Mn3+/oxygen non-stoichiometry, largely affect the electrochemical performance of both Li2MnO3 and lithium-rich manganese-rich (LMR) layered oxides with Li2MnO3 as one of the key components. Herein, Li2MnO3 samples with different amount of structural defects of Mn3+/oxygen non-stoichiometry are prepared. The results clearly demonstrate that the annealed Li2MnO3 (ALMO), quenched Li2MnO3 (QLMO), and quenched Li2MnO3 milled with Super P (MLMO) all show pure C2/m monoclinic phase with stacking faults. MLMO shows the largest amount of Mn3+, followed by the QLMO and then the ALMO. The increased amount of Mn3+ in Li2MnO3 (such as sample MLMO) facilitates the activation of Li2MnO3 and leads to the highest initial discharge specific capacity of 167.7mAhg−1 among the samples investigated in this work. However, accelerated activation of Li2MnO3 also results in faster structural transformation to spinel-like phase, leading to rapid capacity degradation. Therefore, the amount of Mn3+ needs to be well controlled during synthesis of LMR cathode in order to reach a reasonable compromise between the initial activity and long-term cycling stability. The findings of this work could be widely applied to explain the effects of Mn3+ on different kinds of LMR cathodes.