This study presents a comparative analysis of the effects of Ce addition (1 wt%) on the rate-dependent deformation instability pattern of a Direct Chill (DC)-cast Al-5Mg, thoroughly linked to and explained by the corresponding phase and microstructural evolution. Microscopy results revealed the formation of a considerable volume of Ce-rich eutectic intermetallics (primarily of Al11Ce3 and Al13CeMg6 types) at higher temperatures, effectively suppressing the formation of the beta-Al3Mg2 eutectic phase towards the final stages of solidification, a pattern further confirmed via Scheil simulations. Electron Backscatter Diffraction (EBSD) analysis revealed a coarser and less uniform grain size evolution in the Ce-modified alloy, attributed to the potential Ce-Ti reaction and, therefore, the neutralization of Ti-rich grain refiners used in the melt. Furthermore, the Ce-added alloy exhibited a stronger texture development in the as-cast state, primarily stemming from the reduced effectiveness of grain refiners and the pinning and stabilizing effect of Ce-rich intermetallics emerging alongside the alpha-Al primary phase crystallizing during solidification. The analysis of tensile deformation at various strain rates (10- 4 to 10- 2 s- 1) revealed the development of distinct instability patterns, as characterized by the evolution of Portevin-Le Chatelier (PLC) bands. While similar types of bands were observed at each strain rate in both alloys, the Ce-modified alloy exhibited higher stress-drop magnitudes and lower frequencies of band formation, generally indicating a more localized strain development. Meanwhile, the Ce-added alloy exhibits a significantly higher critical strain, epsilon c (strain at the onset of flow instability, 8.15 % vs 2.62 %) at the lower strain rate of 10- 4 s- 1, thereby enabling extended uniform deformation and thus enhancing formability potential for sheet metal forming processes where flow stability and surface quality are critical (e.g., in stamping applications for automotive body panels).