It was known that it is challenging to join lapped aluminum and bare steel with cold metal transfer (CMT) process because of the formation of significant brittle intermetallics. In this study, another attempt was made to join aluminum AA6061-T6 and bare mild steel with CMT plug welding. Welding tests were performed and the bonding mechanisms, fracture modes, and strengths of CMT plug welded joints were systematically characterized. It was found that it is feasible to join 1 mm thick bare mild steel-to-1 mm thick aluminum AA6061-T6. The material stacking sequence and the presence of a predrilled hole significantly affected the weldability of CMT plug welding bare mild steel-to-aluminum AA6061-T6. By positioning bare steel with a predrilled hole in the top aluminum AA6061-T6 and aligning a torch in the center of an 8 mm hole improved significantly the weld appearance and joint strength.