The objective of this study was to examine the impact of varying magnesium levels in the α-Al + S + T region of the Al-Cu-Mg ternary phase diagram on the solidification process, microstructure development, tensile properties, and precipitation hardening of Al-Cu-Mg-Ti alloys. The outcomes indicate that alloys with 3% and 5% Mg solidified with the formation of binary eutectic α-Al-Al2CuMg (S) phases, whereas in the alloy with 7% Mg, the solidification process ended with the formation of eutectic α-Al-Mg32(Al, Cu)49(T) phases. Additionally, a significant number of T precipitates were noticed inside the granular α-Al grains in all alloys. In the as-cast condition, the 5% Mg-added alloy showed the best combination of yield strength (153 MPa) and elongation (2.5%). Upon T6 heat treatment, both tensile strength and elongation increased. The 7% Mg-added alloy had the best results, with a yield strength of 193 MPa and an elongation of 3.4%. DSC analysis revealed that the increased tensile strength observed after the aging treatment was associated with the formation of solute clusters and S″/S′ phases.