Abstract: With the rapid development of 3C industries, the demand for high-thermal-conductivity magnesium alloys with high mechanical performance is increasing quickly. However, the thermal conductivities of most common Mg foundry alloys (such as Mg–9wt%–1wt%Zn) are still relatively low. In this study, we developed a high-thermal-conductivity Mg–4Al–4Zn–4RE–1Ca (wt%, AZEX4441) alloy with good mechanical properties for ultrathin-walled cellphone components via high-pressure die casting (HPDC). The HPDC AZEX4441 alloy exhibited a fine homogeneous microstructure (average grain size of 2.8 μm) with granular Al₁₁RE₃, fibrous Al₂REZn₂, and networked Ca₆Mg₂Zn₃ phases distributed at the grain boundaries. The room-temperature thermal conductivity of the HPDC AZEX4441 alloy was 94.4 W·m^–1·K^–1, which was much higher than 53.7 W·m^–1·K^–1 of the HPDC AZ91D alloy. Al and Zn in the AZEX4441 alloy were largely consumed by the formation of Al₁₁RE₃, Al₂REZn₂, and Ca₂Mg₆Zn₃ phases because of the addition of RE and Ca. Therefore, the lattice distortion induced by solute atoms of the AZEX4441 alloy (0.171%) was much lower than that of the AZ91D alloy (0.441%), which was responsible for the high thermal conductivity of the AZEX4441 alloy. The AZEX4441 alloy exhibited a high yield strength of ~185 MPa, an ultimate tensile strength of ~233 MPa, and an elongation of ~4.2%. This result indicated that the tensile properties were comparable with those of the AZ91D alloy. Therefore, this study contributed to the development of high-performance Mg alloys with a combination of high thermal conductivity, high strength, and good castability.