The impact of Mn addition on the microstructure, mechanical properties and corrosion behavior of T6-treated Al−Si−Mg−xMn (x=0.2−1.0 wt.%) alloys in a 3.5 wt.% NaCl solution was investigated. The results showed that adding 0.2 wt.% Mn to T6-treated Al−Si−Mg alloys enhanced the corrosion resistance by promoting the formation of α-AlFeMnSi phase, characterized by smaller absolute Volta potential values compared to eutectic Si, β-AlFeSi and π-AlFeMgSi phases. However, the addition of 0.5 wt.% Mn and 1.0 wt.% Mn to the T6-treated Al−Si−Mg alloys increased the size of the α-AlFeMnSi phase. This decreased the properties of T6-treated Al−Si−Mg alloys. Therefore, the optimum Mn content was 0.2 wt.%, providing a novel approach for synergistically enhancing mechanical properties and corrosion resistance of Al−Si−Mg alloys.