This work was planned to modify the microstructure of a solution-treated, cast Al-Si-Mg aluminum alloy by a plastic deformation method at a cryogenic temperature. It was found that cryo-rolling is an efficient low-temperature, plastic-deformation method that causes the transformation of a dendritic microstructure to an ultrafine-grained counterpart with a high dislocation density and the redistribution of hard silicon particles in the cast aluminum alloy. The results show cryo-rolling strains lead to an increment of the dislocation density because of the annihilation of the dislocations’ dynamic recovery. The microstructural refinement imposed by cryo-rolling seems to lead to a notable strength enhancement of the material because of the coupled impact of dislocation-strengthening and grain-boundary-strengthening mechanisms.