This study demonstrates that introducing multidimensional crystallographic defects through severe plastic deformation may overcome the strength-ductility trade-off in magnesium alloys. We developed a unique hyper-substructure (HSS) microstructure via novel upsetting-assisted asymmetric extrusion at low temperature (~220 ℃). The UE-55 specimen with ~0.34 upsetting strain exhibited exceptional properties: ~266.5 MPa yield strength, ~311.9 MPa ultimate tensile strength, and ~27.6% fracture elongation in a low-alloyed magnesium system (~3.3 wt.% total alloy content). Remarkably, HSS simultaneously enhances both strength and ductility. Refined grains and elevated dislocation density within HSS primarily strengthen the material. Enhanced plasticity stems from synergistic mechanisms. Pre-existing dislocations multiply during tension through interaction-mediated processes, facilitating c-axis deformation. Simultaneously, linearly aligned low-angle boundaries (LABs) obstruct the propagation of microcracks initiated near high-angle boundaries (HABs) by fractured coarse secondary phases. This significantly improves the material’s microcrack accommodation capacity. This work establishes substructures as primary carriers of plastic deformation, diverging from conventional rapid extrusion techniques that produce fully recrystallized microstructures. The resultant strength-ductility synergy emerges from coordinated strengthening mechanisms. Notably, processing at 20.6 m/min extrusion speed enables efficient fabrication of high-performance magnesium extrudates. Furthermore, analysis of HSS formation mechanisms provides novel insights for industrial-scale production of cost-effective magnesium alloys.