Joining steel plates and aluminium plates by means of using Fe/Al structural (Triclad) transition joints has been well-received in the shipbuilding industry, but is rarely applied in other industries. In this work, 12.7mm thick plates of advanced high-strength steel and AA 6061 alloy were successfully joined by using hybrid laser-arc welding with the help of Triclad transition joints. The parameters were optimized for welding dissimilar steels (A516 to advanced high-strength steel) and welding dissimilar aluminium alloys (AA 5456 to AA 6061). The optimization controlled the total heat input and kept the Al/Fe interface of the Triclad transition joint below the maximum allowable temperature of 315.56°C in order to minimize the growth of brittle intermetallic phases and retain the mechanical properties of the Triclad transition joint. A finite element model was developed to study the temperature evolution at the Triclad interface and the heat distribution along the welded structure. A “4:1” ratio was used between the width of the Triclad transition joint and the thickness of the webs. This ratio improved the load-bearing property of the Triclad transition joint and compensated for the microcracks and brittle Al-rich intermetallic phases observed at the Triclad interface. Tensile strength of 220MPa was achieved at the welded structure. The fracture occurred at the heat affected zone of the AA 5456-to-AA 6061 weld.