The hybrid structures of aluminum-steel have been increasingly used for body-in-white constructions in order to reduce weight and green gas emissions. Obtaining acceptable joints between steel and aluminum required a better understanding of welding metallurgy and their effects on the resultant mechanical properties as well as the microstructure of the joints. In this research, the fiber laser welding of zero-gap galvanized steel and aluminum alloy in an overlapped configuration was carried out. The influence of heat input on the weld bead dimension, microstructural and mechanical properties of the joints was studied. A detailed study was conducted on the effects of the heat input on the penetration depth, weld width and microstructure of the laser welded dissimilar joints by means of an optical microscopy. A scanning electron microscopy with energy dispersive spectroscopy was carried out to determine the atomic percent of the elements for intermetallic compounds (IMC) occurred at the interface of the aluminum and steel. Microhardness measurement and tensile shear tests were conducted to evaluate the mechanical properties of the galvanized steel to aluminum lap joints. The experimental results showed that the penetration depth and weld width increased with the increase of heat input level. However, in order to limit IMC layer thickness and hardness at the surface of the weld seam and aluminum alloy, iron to aluminum dilution should be restricted by limiting the penetration depth. At lower heat input levels, less brittle IMC formation was formed. Consequently, with limited penetration depths at low heat input levels, up to 520 N tensile shear load achieved, with failures located in the interface of the joints.