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Mg-Gd-Y-Mn-Sc alloys with different Mn + Sc contents were prepared by hot extrusion and ageing treatment. The effect of Mn + Sc additions on the microstructures, tensile properties, and corresponding strengthening mechanisms of the as-extruded and peak-aged Mg-Gd-Y-Mn-Sc alloys were systematically investigated. Mn + Sc additions had outstanding grain refinement effect on the homogenized alloy and promoted the supersaturation of the Gd and Y atoms in α-Mg matrix, facilitating the dynamic recrystallization (DRX) and dynamic precipitation during hot extrusion. The DRX mainly occurs via discontinuous DRX (D-DRX) and continuous DRX (C-DRX) mechanisms. The as-extruded alloy with higher Mn + Sc contents exhibits better strength-ductility balance with a tensile yield strength of 337 MPa, ultimate tensile strength of 406 MPa and elongation to failure of 10.8%, which is mainly attributed to higher DRX fraction and denser dynamically precipitated particles. The Mn + Sc additions benefits the age-hardening response and notably improves the tensile yield strength to be 484 MPa for the peak-aged alloys by ageing precipitation and GB solute segregation, which is superior than previously reported Mg-RE-Mn alloys.