Interfacial layers greatly influence the performance of steel–aluminum friction stir welding (FSW) joints, and understanding the formation and evolution of intermetallic compounds (IMC) can help improve the mechanical properties of the welds. In this study, FSW was used to join DP 1180 high-strength steel to 7075 Al at different welding speeds. The effect of the galvanized layer on the IMC formation and evolution, and the mechanical performance of the steel–Al FSW joints were investigated. It was found that the galvanized steel–Al joints were formed only by metallurgical bonding, a continuous IMC layer composed of FeAl, Fe3Al, and Al–Zn eutectic developed at the joint interfaces. Joints were mechanically and metallurgically bonded in the non-galvanized steel, and a 3 µm thick IMC layer consisting of FeAl existed only in the stir zone (SZ). IMC layer formation was predicted according to thermodynamic principles, which is consistent with the interfacial microstructure evolution identified by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Shear tensile test results showed that the galvanized layer can effectively improve the metallurgical bonding strength of the steel–Al joints, and the optimum tensile properties were found in galvanized steel–Al joints.