The effects of phase content and evolution on mechanical properties of the alloys are investigated by extruding Mg95Y2.5Zn2.5 and Mg93.1Y2.5Zn2.5Ti1.6Zr0.3 alloys with different original microstructures. The 18R-LPSO phase plays a decisive role in the strength of as-extruded alloys. The spherical W-MgYZn2 phase enhances the deformability and ductility of the alloys. The dynamic recrystallization grains can be observed in all as-extruded alloys. However, the 14H-LPSO clusters and dynamic precipitates only form in the as-cast and T41-treated (solid-solution treatment at 530 °C for 3 h with water cooling) alloys after extrusion. Based on synergistic effects of these phases mentioned above, the T41-treated Mg93.1Y2.5Zn2.5Ti1.6Zr0.3 alloy exhibits good compressive mechanical properties after extrusion.