In this study, we investigate the effects of longitudinal magnetic field on the welded joint formation characteristics, ferrite/austenite microstructures, and fatigue crack propagation in the laser-MIG welding of 316L austenitic stainless steels. The experimental results indicate that the upper hybrid layer exhibits shallower penetration and larger width, whereas the lower hybrid layer exhibits a better symmetry with the addition of the magnetic field. Further, the melting area of the two hybrid layers and the remelted area between the two hybrid layers remain almost constant. The agitation of the solid-liquid front of the molten pool is promoted by the addition of the magnetic field, resulting in different fusion line shape characteristics. Furthermore, the addition of the magnetic field changes the growth direction of the microstrucuture near the fusion line, improves the thermal cycle of the base metal near the fusion line, reduces the width of the heat-affected zone, and restrains the coarsening of the grains. The magnetic field can promote the transition of cellular grains to the dendrites in the interlayer, change the ferrite dendritic morphology, and refine the interlayer and intralayer austenite structures. The fatigue crack growth resistance increases owing to the refinement of the microstructure, reducing the crack sensitivity of the joint.