In the context of global industrial green transformation and the demand for light-weighting, magnesium alloys, as the most promising lightweight engineering materials, still face limitations due to inherent poor ductility, low strength, and insufficient corrosion resistance. ZM6 (Mg-Nd-Zr) magnesium alloy has attracted significant attention for its notable age-strengthening response; however, the quality of its welded repairs remains a critical bottleneck hindering its engineering application. Results demonstrate that ultrasonic-assisted welding significantly refines weld grain size (average 10–15 μm) and reduces porosity via acoustic streaming and cavitation effects. Pre-weld heat treatment effectively activates precipitation strengthening, promoting the uniform and dispersed precipitation of strengthening phases such as Nd-enriched secondary phases. Consequently, the mechanical properties of the repaired joint are remarkably restored, achieving an ultimate tensile strength of 267 MPa, a yield strength of 210 MPa, and an elongation of 6.8%. The microhardness of the repaired zone recovered to 72 HV, approaching the base metal value (83 HV). Fractography revealed a ductile–brittle mixed-fracture mode. This research confirms that the combination of ultrasonically assisted TIG welding and pre-weld heat treatment is a viable strategy for effectively enhancing the repair quality of ZM6 magnesium alloy, enabling the synergistic recovery of strength and ductility, thereby providing important theoretical and practical significance for promoting the reliable application of magnesium alloys in high-end equipment manufacturing.