Mechanical properties of fiber laser welded (FLWed), friction stir welded (FSWed), and friction stir spot welded (FSS weld) AZ31B-H24 Mg and Al 5754 alloys were studied. After welding, grains at the weld centre became recrystallized. β-Mg17A112 particles appeared in the fusion zone of the joints during laser welding, while a characteristic interfacial layer consisting of A112Mg17 and Al3Mg2 was observed in the A1/Mg dissimilar FSS weld. In FLWed joints, a joint efficiency of ~91% with superior yield strength, ultimate tensile strength and fatigue strength was achieved at a higher welding speed. In FSWed joints, a higher welding speed of 20 mm/s and lower rotational rate of 1000 rpm led to higher YS, but lower ductility, strain-hardening exponent and hardening capacity. In FSS weld joints, Mg/Mg, A1/A1 FSS welds and Al/Mg adhesive, Mg/A1 adhesive FSS welds had a significantly higher lap shear strength and fatigue life than the A1/Mg FSS weld.