Abstract: The effects of adding Mn to the Mg–15.0Zn (wt%) alloy, prepared using high-pressure die casting, on the microstructure, mechanical properties, thermal conductivity, and plastic deformation mechanisms of the alloy were investigated. The results indicated that the Mn addition reduced the grain size of the alloy from 6.82 to 5.22 μm, while slightly coarsening its eutectic phases, improved its casting properties, and eliminated the cracks generated during the casting process. Transmission electron microscopy and quasi-in situ electron backscatter diffraction results suggested that the addition of Mn had activated prismatic and <c + a> slips in the alloy during its tensile deformation. Moreover, mechanical test results showed that the addition of Mn had significantly improved the ultimate tensile strength of the alloy: from 287 MPa for the Mg–15.0Zn (wt%) alloy to 317 MPa for the Mg–15.0Zn–0.3Mn (wt%) alloy. Meanwhile, the thermal conductivity of the alloy increased from 68.9 to 78.6 W·K⁻¹·m⁻¹ following the addition of Mn, which reduced the content of Zn atoms. This study will provide insights into the plastic deformation mechanisms of ultrahigh-Zn die-cast Mg alloys.