The effects of microalloying with Mg (0–0.23 wt%) on the microstructural evolution and mechanical properties of Al–Cu 224 cast alloys at ambient and elevated temperatures are investigated using transmission electron microscopy, differential scanning calorimetry, and tensile/compression testing. The results show that microalloying with Mg significantly enhances the precipitation of the θ′ phase during aging, producing fine, dense, and uniformly distributed θ′ precipitates. These precipitates are much more effective for alloy strengthening than are the θ″ precipitates in the alloy without Mg. During stabilization at 300 °C for 100 h, the dominant process becomes coarsening of the θ′ phase. The Mg-containing alloys have much finer and denser θ′ precipitates and thus considerably higher yield strengths at elevated temperature as compared to those of the alloy without Mg. The improvement is more pronounced at low Mg contents (0.09%–0.13%) than at high contents. The yield strength at 300 °C of the 0.13% Mg alloy is as high as 140 MPa, which is far superior to that of most cast aluminum alloys. Moreover, the enhanced yield strength of this alloy is well preserved during prolonged exposure at 300 °C for 1000 h, indicating that it is a promising lightweight material for high-temperature applications. Display omitted •Effect of Mg on microstructure and room-/high-temperature strength of Al–Cu alloy.•Enhanced precipitation of the θ′ phase by microalloying with Mg.•Upon exposure at 300 °C for 100 h, the best characters of θ′ is obtained at 0.13% Mg addition.•Strength at 300 °C of Al–Cu alloy with 0.13% Mg highly exceeds that of most Al alloys.•Enhanced strength of alloy remains well preserved even after exposure at 300 °C for 1000 h.