The effect of long period stacking ordered (LPSO) phase and γ' precipitates on the ageing behavior and mechanical properties of the extruded Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy was investigated. The results show that more β' phases precipitate during ageing treatment in the LPSO phase containing alloy so that the LPSO phase containing alloy exhibits a higher age-hardening response than the γ' precipitates containing alloy. The precipitation strengthening induced by β' precipitates is the greatest contributor to the strength of the peak-aged LPSO-containing alloys. Higher strength is achieved in γ' precipitates containing alloy due to the more effective strengthening induced by dense nanoscale γ' precipitates than LPSO phases as well as the higher volume fraction of coarse unrecrystallized grains with strong basal texture. The extruded alloy containing γ' precipitates after T5 peak-ageing treatment shows ultra-high tensile yield strength of 462 MPa, high ultimate tensile strength of 520 MPa, and superior elongation to failure of 10.6%.