The effect of the strain-hardening rate on the plastic strain distribution in parts made from aluminum alloys by the Splined Mandrel Flow Forming (SMFF) process was investigated. Parts were made from annealed 5052 and 6061 aluminum alloys by SMFF with different levels of average work piece thickness reduction, from 20 to 60%. The average yield stress for the 5052 and the 6061 alloys increased by 187% and 87% after SMFF (60% thickness reduction). The larger increase in yield stress of the 5052 work piece is attributed to its higher strain-hardening coefficient resulting from it containing higher levels of solid-solution Mg compared to the 6061 alloy. While the magnitude of the average von-Mises equivalent plastic strain through the thickness of the SMFF work piece was essentially the same for both alloys, the local equivalent plastic strain near the surface of the work piece and the point-to-point scatter in the measured plastic strain was greater in the 5052. This is attributed to the effect of the increased solid-solution Mg content localizing plastic flow. These findings illustrate the role of solid solution strengthening additions, in this case Mg, on increasing the average mechanical strength but also increasing the extent of local plastic strain variability in aluminum alloy material subjected to intensive plastic forming operations such as SMFF.