The effects of the quench temperature (cooling rate) and melt-holding time at high temperatures on the precipitation kinetics and nanostructure of the Al–Si–Mg alloy were investigated by measuring the hardness and electrical conductivity, differential scanning calorimetry, transmission electron microscopy, and atom probe tomography. The results showed that a high quenching temperature (high cooling rate) and long melt holding simultaneously increased the hardness and electrical conductivity of the Al–Si–Mg alloy. They also accelerated the overall precipitation kinetics; however, the effect was more evident when the duration of high-temperature melt holding increased. Nanostructural analysis of the peak-aged conditions revealed that the accelerated precipitation kinetics of the alloy with a high quenching temperature were primarily related to the high amount of Si solutes, which prevented the formation of coarse Si precipitates. Conversely, the high precipitation kinetics caused by longer melt-holding treatments depend on the increased amount of solutes and dissolution of clusters and could disturb the formation of strengthening β″ precipitates. These results indicate that the mechanical properties of the T5-treated Al–Si–Mg alloy can be significantly enhanced by increasing the quenching temperature and providing sufficient melt holding at high temperatures.