This study investigates the effects of the extrusion ratio and annealing treatment on the microstructure, texture and mechanical properties of an as-extruded Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr (wt%) alloy. A high extrusion ratio (30:1) results in a homogeneous microstructure with fine dynamic recrystallized (DRXed) grains, while a low extrusion ratio (6:1) leads to a bimodal microstructure with un-DRXed regions and DRXed grains. The bimodal microstructure can be removed by subsequent annealing. This alloy contains several long-period stacking ordered (LPSO) and Mg5RE phases (RE: rare earth). The extrusion ratio and annealing process have negligible effects on the volume fraction of the LPSO phase but have significant effects on the Mg5RE phase. The volume fraction of the Mg5RE phase decreases as the extrusion ratio and annealing time increase. Cuboid precipitates form in the alloy extruded at low extrusion ratios after annealing. The alloy exhibits a bimodal texture with < 0001 > and < 1010 > components. The presence of the < 0001 > component is determined by a critical grain size. The texture evolution (such as the degree of grain growth) is not influenced by the extrusion ratio, but it is affected by the annealing time, which is related to the Mg5RE phase rather than the LPSO phase. The grain refinement, the Mg5RE and LPSO phases, and the texture contribute to the alloy strengthening. Finally, a high-strength extruded Mg bar with a diameter of 32 mm (an extrusion ratio of 6:1) was successfully produced.