In this study, a thermodynamic description of the Mg-Mn-O system is presented. Phase diagram data, structural information, and thermochemical data are used in the assessment. All solution phases are modeled using the compound energy formalism. In addition, heat capacity measurement of the MgMn2O4 and Mg6MnO8 phases are reported. The thermodynamic description reproduces thermochemical, structural, and phase diagram data.