The effects of pre-ageing treatment on the microstructure and mechanical properties of the Mg-9.2Gd-4.4Y-1.0Zn-0.8Mn (wt.%) alloy were investigated. Microstructural analysis indicated that the ageing treatment before extrusion led to the formation of dense prismatic β′ and basal γ′ precipitates in heat treated alloys. The presence of these precipitates and their solution obstructed the dynamic recrystallization process during hot extrusion. The lamellar long-period stacking ordered (LPSO) phases restrained the recrystallization through forming the kink band and releasing the stress concentration, and the fine β-Mg₅(Gd, Y) particles suppressed the recrystallization by the particle pinning effect. The block-shaped LPSO phases and coarse β-Mg₅(Gd, Y) particles promoted the recrystallization following the particle stimulated nucleation (PSN) mechanism. The combined effects led to the formation of the bimodal microstructure, which shows fine recrystallized grains with random grain orientation and deformed grains with strong fiber texture. The bimodal microstructure with lower recrystallization fraction provides the alloy higher strength and lower ductility. With solid-solution and pre-ageing treatments, the as-extruded alloy shows the best strength-ductility balance with an ultimate tensile strength (UTS) of 455 MPa, tensile yield strength (TYS) of 382 MPa and elongation to failure (EL) of 11.0%. The outstanding mechanical properties are mainly attributed to the bimodal microstructure, strong fiber texture, β-Mg₅(Gd, Y) particles, lamellar and block-shaped LPSO phases.