We report on the synthesis and characterization of a new metastable polymorph of Li₂MnSiO₄ adopting the Pn space group, prepared by ion-exchange from Na₂MnSiO₄. Density-functional theory methods were used to predict the lattice parameters and atom positions of the new polymorph material and those of Na₂MnSiO₄ and LiNaMnSiO₄, allowing their identification by X-ray diffraction profiles, as well as the comparison of the measured and calculated cell parameters. The electrochemical activity of this new polymorph as a cathode material for lithium ion batteries was evaluated in coin cells and compared to that of the thermodynamically stable Pmn2₁ polymorph of Li₂MnSiO₄, as well as LiNaMnSiO₄ and Na₂MnSiO₄. Carbon coating, very vital to the electrochemical activity of the material, was added in situ to the material before ion-exchange because the metastable polymorph converts to the stable polymorph above 370 °C, as confirmed by differential scanning calorimetry scans. Both Li₂MnSiO₄ polymorphs display similar charge–discharge curves, except for a marginally lower lithium extraction voltage during the first charge of the Pn structure that may be due to the presence of sodium ion impurities. A discharge capacity of 110 mA h g^–1 is initially observed for both Li₂MnSiO₄ polymorphs and both exhibit similar capacity fades. LiNaMnSiO₄, however, yields a stable capacity of 45 mA h g^–1, whereas Na₂MnSiO₄ yields an initial capacity of 20 mAh g^–1, increasing to 60 mAh g^–1 with cycling.