Li2MnO3 has received lots of attention due to its large theoretical capacity. However, its application is obstructed by low practical capacity and poor cycling stability. Here, a new tactic of magnesium substitution for partial lithium to improve the electrochemical performance of Li2MnO3 by a traditional solid state reaction is presented. Sample with only 1% magnesium content delivers a large initial discharge capacity of 307.5 mAh g−1 which is much superior than the 241.9 mAh g−1 of pristine Li2MnO3. The cycling performance also get improved that the capacity retention is 84.5% for Li1.98Mg0.01MnO3 but only 77.7% for pristine counterpart after 30 cycles at 0.1 C. Moreover, the effective suppression of voltage decay has been achieved because of improved kinetic properties and leads to a tremendous progress in the energy density which is as much as 944 Wh kg−1 for Li1.98Mg0.01MnO3 compared with 747.1 Wh kg−1 of Li2MnO3. These achievements attained by Mg-doping make Li2MnO3 a promising positive-electrode material for the next generation of lithium ion batteries.