AbstractA wide application of magnesium (Mg) alloys in electronic devices requires high thermal conductivity and mechanical properties simultaneously. Unfortunately, the strategies for increasing their strength, such as alloying, decrease their thermal conductivity. Aging treatment is an effective method to resolve the strength-thermal conductivity trade-off. In this study, the mechanical properties and thermal conductivity of previously designed as-cast Mg–Zn–La/Ce alloys were further improved by aging treatment. Mg–Zn–La/Ce alloys aged at 473 K for 20 h exhibit a good comprehensive property. The thermal conductivity, yield strength, and ultimate tensile strength of aged Mg97.3Zn2.6La0.1 alloy and Mg97.4Zn2.5Ce0.1 alloy reach 148.4 W/(m·K), 172.9 MPa, 218.7 MPa, and 155.3 W/(m·K), 172.1 MPa, 239.1 MPa, respectively. The effects of La/Ce addition and precipitation on thermal conductivity were analyzed from the aspects of intermetallic phases, precipitates, and solid solutions using transmission electron microscopy (TEM) observations and precipitation kinetic calculations. The addition of La or Ce has a positive effect on the thermal conductivity due to the formation of ternary τ1 phase which reduces the supersaturation of the Mg matrix. The precipitation shows two opposite influences on thermal conductivity. The sharp reduction in supersaturation of the matrix enhances the thermal conductivity, but the dense precipitates hinder the free movement of electrons and phonons.