AZ61 Mg alloy to Q235 mild steel were lap joined using hybrid laser-TIG welding technique. At the joint interface and fusion zone (FZ), microstructure was revealed by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy; element distribution was analyzed by electron probe micro-analyzer; intermediate phases were identified using X-ray diffraction test. Comparing with interlayer-free joints, the new intermediate phases Mg2Ni and Mg2Cu were generated in the FZ and at the Mg alloy/interlayer interface, and the solid solution of Ni or Cu in Fe was found along the edge of weld pool on steel side. It was found that direct joint without any interlayer was mechanical bonding, while Ni- and Cu-added joints were semi-metallurgical bonding. The joint shear strength was not only related to the penetration depth, but also related to the bonding mode. The strengthening effect on Cu-added joint was higher than that on Ni-added joint.