The effect of lanthanum (La)+cerium (Ce) addition on the high-temperature strength of an aluminum (Al)–silicon (Si)–copper (Cu)–magnesium (Mg)–iron (Fe)–manganese (Mn) alloy was investigated. A great number of plate-like intermetallics, Al11 (Ce, La)3-and blocky α-Al15 (Fe, Mn)3Si2-precipitates, were observed. The results showed that the high-temperature mechanical properties depended strongly on the amount and morphology of the intermetallic phases formed. The precipitated tiny Al11 (Ce, La)3 and α-Al15 (Fe, Mn)3Si2 both contributed to the high-temperature mechanical properties, especially at 300◦C and 400◦C. The formation of coarse plate-like Al11 (Ce, La)3, at the highest (Ce-La) additions, reduced the mechanical properties at (≤300) °C and improved the properties at 400 °C. Analysis of the strengthening mechanisms revealed that the load-bearing mechanism was the main contributing mechanism with no contribution from thermal-expansion mismatch effects. Strain hardening had a minor contribution to the tensile strength at high-temperature.