<p>This study addresses the effect of the trace Nb addition in 357 cast alloy and its subsequent role in driving the aging and precipitation kinetics. The variations in microstructure and mechanical properties are analyzed with a primary focus on restoring the ductility in over age state without compromising the strength characteristics. The prepared cast alloys: (i) N-0 (without Nb) and (ii) N-1 (containing a trace 0.035 wt% of Nb) display comparable optical microstructure and Si eutectic phase characteristics. The microstructural (TEM) analyses and comparison of numerical features of the precipitates reveal that Nb addition has significantly stimulated the precipitation kinetics by generating finer and denser precipitates. As the age hardening response is determined by the precipitation kinetics; therefore, the N-1 alloy exhibits a quicker and better age hardening response and superior thermal endurance as compared to the N-0 alloy both before and after the peak aging. Moreover, the trace Nb addition has also reduced/delayed the over aging effects by suppressing the further growth and transformation of the Mg₂Si precipitates (from β′′ to β′). The tensile properties indicate that the ductility is abruptly decreased for N-0 alloy, with marginal improvement in strength characteristics from peak to over age conditions. On the other hand, the trace Nb (0.035 wt%) addition in N-1 alloy not only maintains and restores the ductility values in both peak and over age conditions, respectively, but also further improves the strength properties as well. The fractographic microstructures of tensile surfaces are logically aligned with the obtained mechanical responses and microstructural features of both alloys.</p>