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 exhib-ited 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. There-fore, this study contributed to the development of high-performance Mg alloys with a combination of high thermal conductivity, high strength, and good castability.