This paper analyses the influence of thermal-induced aging on the strength of cast A319 and A356 aluminum alloys. This phenomena is of primary importance especially in the automotive industry, where on one hand the alloys experience Thermo-Mechanical and Low Cycle Fatigue loading conditions and on the other hand their microstructure is induced both by the manufacturing process and the thermal histories. The alloys studied here were produced using an industrial Lost Foam Casting process which affects significantly the microstructure and the precipitation compared to standard Die Casting process. The paper compares mechanical properties of these peculiar alloys in terms of microhardness at the macroscopic level as a function of thermal overaging time and at the microscopic level in terms of properties of the different phases (intermetallics, eutectics) present in the microstructure. Important differences can be observed between these different phases and the results are consistent with similar studies from the literature on different aluminum alloys and processes.