Mg alloys are extensively used as light structural materials in the modern industry. Mn and RE (rare earth) metals are effectively used as alloying elements to enhance the properties of Mg alloys. Therefore, the thermodynamic database of those alloys is important in the design and processing of Mg alloys. Critical evaluation and optimisation of all available literature data for the Mn-Y and Mn-Gd systems were conducted to obtain reliable thermodynamic functions of all solid and liquid phases. Recent experimental data were taken into account and the phase diagram and thermodynamic properties were slightly revised for the Mn-Y system and considerably altered for the Mn-Gd one from previous studies. Both systems have quite similar thermodynamic properties and phase diagrams. The systematic trend observed in RE elements for Mn-RE systems is discussed.