<p>Grain refinement is an effective means to improve the properties of magnesium alloys. In this paper, the effects of synergistic addition of Mn and Al elements on grain refinement and mechanical properties of Mg-10Gd-3Y alloy were studied. A significant elevation in Mn and Al concentrations to 0.9 wt.% resulted in marked grain refinement, effectively diminishing the mean grain size from 159 ± 7 to 72 ± 2 μm. The original coarse dendritic microstructure was transitioned into fine equiaxed grains. The second phase in Mg-10Gd-3Y alloy is predominantly reticulated Mg-RE phase. The incorporation of Mn and Al facilitates a decrease in reticular Mg-RE phase, forming granular and need-like Al₂RE phase. Coherent with the α-Mg matrix, the granular Al₂RE phase serves as a potent heterogeneous nucleation site that promotes grain refinement. A substantial enhancement in mechanical properties was achieved through the Mn and Al addition. With the content at 0.9 wt.%, it attained a yield strength of 167 MPa, an ultimate tensile strength of 200 MPa, and an elongation of 7.4%, corresponding to impressive improvements of 61%, 60%, and 90%, respectively, compared to the unreinforced matrix alloy. This superior comprehensive property stems from the synergistic strengthening contributions of grain refinement and the second phase.</p>