Significantly improving the absolute strength of low-cost wrought magnesium alloys is one of the key bottlenecks in the field of light alloy research. In this study, a new Mg–Sm–Ce–Mn alloy containing low content of light rare earth elements was designed based on phase diagram calculation, and an ultra-high strength of nearly 400 MPa was achieved by low temperature extrusion. The Mg–0.8Sm–0.4Ce–0.4Mn (EM10, mass fraction, %) alloy formed a  dual-configuration grain structure after low temperature extrusion, and showed a typical silk texture, and dynamically precipitated Mg₄₁(Ce, Sm)₅ and Mg₁₂Ce micron second phase. The lower extrusion temperature and the precipitation of high-density nano-second phases contributed to the refinement of the recrystallized grain size of EM10 alloy to about 0.93 μm, while a high density of residual dislocations remained in the uncrystallized region. The combined effects of fine grain strengthening, second phase strengthening, texture strengthening and dislocation strengthening lead to the yield strength of EM10 alloy can be as high as about 379 MPa, while maintaining the elongation of about 4.7%. Continuing to increase the content of Ce and Sm elements, more brittle micron-sized second phases will be introduced, which will impair the plasticity of the alloy. Therefore, a high-strength Mg–0.8Sm–0.4Ce–0.4Mn alloy was designed and prepared by low-temperature extrusion process and composition optimization. The relevant results can provide theoretical guidance for the preparation of low-alloying and ultra-high-strength wrought magnesium alloys.