Mechanical clinching method has been shown to be a feasible alternative to self piercing riveting (SPR) for joining dissimilar materials, particularly for vehicle assemblies made in aluminum-to-steel. While a great deal of effort has been focused on studying the process development, there is an urgent need to understand the effects of aged aluminum on the strengths of the clinching steel-aluminum joints. To evaluate exactly the influence of mechanical properties of aged aluminum alloy on the strength of the clinching joint, 1.0 mm thick AA6111-T4 aluminum was firstly solution heat-treated at 550 °C for 30 min followed by nature ageing treatment for various times and then clinching to 0.7 mm thick galvanized SAE1004 steel with SKB die. The section parameters and strength of the clinching joints were measured. The effect of mechanical properties of aged AA6111-T4 aluminum was analyzed using an analytical model. The results showed that aged aluminum affected little the clinchability but influenced the section parameters and strength of the clinching workpieces. The strength of the clinching SAE1004-to-AA6111-T4 joints increased with increasing the strength of AA6111-T4 aluminum. This can be primarily attributed to the fact that the strengthening AA6111-T4 aluminum resulted in larger residual press stress developed during the clinching.