<p>Magnesium–steel joints were fabricated using ultrasonic welding technology. This study examined the effect of welding time on interface formation, microstructure, mechanical properties, and fracture modes of the joints under varying welding durations. The results indicate that with increasing welding time, the interface morphology evolved from slight undulations to a relatively flat structure, which later developed into a wavy form featuring mechanical interlocking. Scanning electron microscopy and X-ray diffraction analyses showed no distinct intermetallic compounds at the magnesium/steel interface; only α-Mg and α-Fe phases were observed. As welding time increased, interfacial bonding strength gradually improved, leading to a corresponding enhancement in joint strength, which peaked at approximately 2298 N. However, joint strength declined when the residual thickness of the magnesium plate decreased to 0.48&#xa0;mm (32% of its original thickness). This thinning resulted in stress concentration and crack initiation, ultimately weakening the joint. Concurrently, the fracture mode shifted from predominantly brittle fracture to a brittle–ductile mixed mode.</p>