Simultaneous achievement of defect-free formation and high mechanical performance in additive manufactured Mg alloys remains challenging, bottlenecked by flammability, porosities, and oxidation risks within the melting-solidifying process. Here, wire-based friction stir additive manufacturing (W-FSAM), sparked by continuous wire feeding, severe plastic deformation transport, and solid-state deposition, was exploited to achieve sound Mg components. Greatly refined grains were obtained by severe plastic deformation, with an average grain size of 3.79 µm. Al₈Mn₅ precipitates underwent refinement and redistribution, while Mg₁₇Al₁₂ precipitates experienced solid solution and precipitation behaviors. Lattice mismatch was 4.92 % when Mg₁₇Al₁₂ precipitated on Al₈Mn₅. The strong flow-induced effect of the pin structure weakened the basal texture along the building direction, which was formed by the intense forging effect of W-FSAM tools. Alternating texture features coordinated the plastic deformation of slip and twinning, enabling superior ductility. Additionally, in-situ electron backscattered diffraction results revealed grain boundary sliding accommodated by grain rotation within the fine-grained structure. These unique microstructural features and precipitate behavior enhanced the overall mechanical properties, with an ultimate tensile strength of 257.3 ± 3.5 MPa and an elongation of 12.4 ± 0.3 % in building direction, and an ultimate tensile strength of 250.7 ± 2.0 MPa and an elongation of 12.2 ± 0.5 % in the traveling direction.