The heterogeneity of α-Al(Fe,Mn)Si dispersoids and β″ precipitates was tuned to enhance the strength- ductility synergy of air-cooled Al-Mg-Si alloys. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to elucidate the microstructural parameters of these two strengthening phases. The results show that the microstructural heterogeneity can be triggered by the absence of homogenization, resulting in the presence of dispersoid-free zones (DFZs) and dispersoid zones (DZs), in conjunction with bimodal β″ precipitates. Further analytical calculations, from the strengthening model, clarify that the strategically dispersed α-Al(Fe,Mn)Si and β″ particles create “soft” and “hard” domains within the alloy, resultantly improving the mechanical properties.