In this work, the effects of 0, 0.15%, 0.35%, 0.55% and 0.75% Mg (mass fraction) on the mechanical properties and microstructure of the Al-5Cu-0.7Mn alloy processed by foundry were investigated using microhardness test, tensile test, scanning electron microscope (SEM) and transmission electron microscope (TEM). The evolution of precipitation behavior was discussed, revealing the relationship between yield strength and aging precipitates. The results demonstrate that the aging precipitates of the Al-Cu-Mn-Mg alloys are transformed from the θ′′ phase (Al₃Cu) to the θ′ phase (Al₂Cu), with the appearance of the S phase (Al₂CuMg) and the σ phase (Al₅Cu₆Mg₂) after Mg addition. Mg-added alloys have a wider variety of phases compared to 0% Mg alloys with only the θ′′ phase, which significantly enhances the strength of the alloys. The alloy with 0.55% Mg added has excellent mechanical properties, with yield strength, tensile strength and elongation are 348 MPa, 496 MPa and 14.1%, respectively. Based on the quantitative statistics on the strengthening contribution of the precipitates, the yield strength of the 0.55% Mg alloy increases by 251.7 MPa due to aging precipitates, which is higher than that of the 0% Mg alloy (147.5 MPa). This indicates that the addition of Mg has a positive effect on the enhancement of the comprehensive mechanical properties of Al-Cu-Mn alloy.