Development of reliable magnesium (Mg) to steel joining methods is one of the critical issues in broader applications of Mg in automotive body construction. Ultrasonic spot welding (USW) has been demonstrated successfully to join Mg to steel and to achieve strong joints. In this study, corrosion test of ultrasonic spot welds between 1.6 mm thick Mg AZ31B-H24 and 0.8 mm thick galvanized mild steel, without and with adhesive, was conducted. Adhesive used was a one-component, heat-cured epoxy material, and was applied between overlapped sheets before USW. Corrosion test was conducted with an automotive cyclic corrosion test, which includes cyclic exposures of dipping in the 0.5% sodium chloride (NaCl) bath, a constant humidity environment, and a drying period. Lap shear strength of the joints decreased with the cycles of corrosion exposure. Good joint strengths were retained at the end of 30-cycle test. Failure mode of joints without adhesive was Mg nugget pull-out, but was interfacial failure of samples with adhesive. Metal-metal bonding between Mg and steel was maintained throughout 30 cycles of corrosion exposure. Joint strength of corroded samples without adhesive decreased due to the erosion in the joint and in the parent material. For the samples with adhesive, the joint strength decreased due to the corrosion attack at the joint interface and the delamination of adhesive from Mg sheet. Adhesive was shown to help in minimizing the erosion of the interior surface of Mg sheet.