The phase stability of the Li₂MnSiO₄ during Li insertion/extraction, is a key requirement for acceptable cyclability and practical application as a cathode material for lithium batteries. Here we present first-principles calculations used to study the phase stability of Mg substituted Li₂MnSiO₄. The 137 structures of Li₂Mn_1−xMg_xSiO₄ (x = 0.25–0.50) were calculated based on 5 known polymorphs of Li₂MnSiO₄. Using three different functionals (PBE, PW91 and PBEsol), it is shown that the total-energy vs. distance between layers in the layered Pmn2₁ curve has a clear minimum and does not demonstrate the exfoliation of layers found previously. The amorphlization of Li₂MnSiO₄ is explained by high value of energy above Hull of its fully delithiated form. Crystal orbital Hamiltonian populations (COHP) revealed that the strength of Mn–O and Si–O bonds unchanged during the substitution with Mg, thus eliminating the concern about the safety. The pure Li₂MnSiO₄ in the P2₁/n form was suggested as the most stable upon cycling. In the Mg substituted Li_2−xMnSiO₄ case, the Mg substitution is more beneficial for x in the range (0.0–2.0) than in the range (0.0–1.0). The increase in the performance for the x = 0.0–1.0 region, can be explained by the small particle size and the uniformity of nanoparticles distribution rather than the enhancement of the thermodynamic stability.