<p>Aging behavior and phase transformation of prerolled Al-Mg-Cu(-Si) alloys were investigated via transmission electron microscopy and mechanical testing. The application of prerolling treatment to the alloy can serve to further enhance the alloy’s hardness by introducing dislocations. Subsequent annealing resulted in an increase in the hardness of the Si-containing alloys, attributable to a strong age-hardening effect. By contrast, the Si-free alloys exhibited a decline in hardness. The prerolled Al-3Mg-0.5Cu-0.2Si alloys exhibited superior elongations to those of other alloys, increasing by 2.56%, 2.95%, and 2.1%, respectively. Its yield (207.2 MPa) and tensile (273.96 MPa) strengths were comparable with those of the Al-3Mg-1Cu alloy. This phenomenon can be attributed to the emergence of fine Guinier-Preston-Bagaryatsky II (GPB-II) and C phases that precipitate along the dislocations in prerolled Si-containing alloys. In contrast to the S phase, which grows along the (120) direction in dislocations, the C phase grows along the (001) direction of the Al matrix. Consequently, the C phase can facilitate the retention of a proportion of the long dislocations during annealing. Furthermore, the addition of Si resulted in the transformation of the precipitates within the alloy, reducing the number of coarse S phases growing along the dislocations while enhancing the number of fine GPB-II phases and zones within the alloy. This core-shell GPB-II phase is formed by the growth of the GPB zone around the periphery of the L′ phase. The formation of this core-shell structure is attributable to the differing diffusion rates of the clusters and the differences in thermodynamic parameters of the elements.</p>