© 2019 The age-hardening behavior and mechanical properties of the extruded Mg-2Gd-1.2Y-0.5 Zn (at.%) alloy with Zr or Mn additions were investigated. The results show that Mn added alloy exhibits more remarkable age-hardening response than Zr added alloy, which is attributed to the fact that partitioning of Mn into the β phases, β′ precipitates and long period stacking ordered (LPSO) phases leads to the decrement in (Gd+Y) concentrations in the second phases, facilitating the precipitation during aging treatment. Both peak-aged alloys show bimodal microstructure comprising the fine DRXed grains with nano-sized β phases pinned at DRXed grain boundaries as well as coarse worked grains with strong fiber texture. High strength and good thermal stability were obtained in both peak-aged Mn and Zr added alloys. While the peak-aged Mn added alloy shows higher strength and superior creep resistance due to its denser β′ precipitates, thin LPSO phases and γ′ precipitates and higher area fraction of worked grains with strong fiber texture. The 0.2% tensile proof stress and ultimate tensile strength of peak-aged Mn added alloy reach up to 454 MPa and 508 MPa, respectively, with elongation of 3.2% at room temperature. The minimum creep rate of the peak-aged Mn added alloy at 250 °C / 150 MPa is 2.4 × 10−8 s−1, which is superior than previously reported extruded Mg-Gd based alloys.