Magnesium-substituted lithium manganese orthosilicate (Li2MnSiO4) cathode materials with a nominal composition of Li2Mg x Mn1−x SiO4, for x =0.4 and 0.5 are synthesized by a solid-state route, at 700°C in argon. The samples are characterized using powder X-ray and neutron diffraction, scanning electron microscopy, and galvanostatic cell-cycling. Rietveld analyses of the powder X-ray and neutron diffraction data show the formation of a monoclinic P21/n structure related to gamma lithium phosphate with no significant impurity peaks. This structure of the Mg-substituted samples is in contrast to the unsubstituted Li2MnSiO4 compound that has a Pmn21 structure when synthesized under the same conditions. Unit-cell volumes of the Mg-substituted materials are intermediate between those of the P21/n structure of Li2MnSiO4 and the isostructural low-temperature form of Li2MgSiO4, indicating the formation of a solid solution. The Mg-substituted materials feature mixed Mg/Mn cation sites, although no evidence of Li/Mn, Li/Mg or Li/Mg/Mn mixed sites are found. The Li2Mg x Mn1−x SiO4 cathodes show improved electrochemical performance over that reported for the unsubstituted Li2MnSiO4 P21/n phase. The Li2Mg x Mn1−x SiO4 cathode performance remains limited by its poor electronic properties and the large particle size of the solid-state synthesized products. Optimization of the synthesis conditions is likely to lead to enhanced electrochemical performance.