The application of the Mg-RE alloys in the aerospace field is often limited by their strength-ductility trade-off. In this study, the effect of La₂O₃ with different mass percentages on the microstructure and mechanical properties of the wire arc directed energy deposition (WA-DED) manufactured Mg-Nd-Zn-Zr alloys was investigated. Among 4%, 8%, and 12% La₂O₃ specimens (the mass percentage of La₂O₃ in the La₂O₃-alcohol solution), the 8% La₂O₃ specimens produced a refined, equiaxed grain structure and the best strength-ductility synergy. At room temperature, the 8% La₂O₃ specimens exhibited excellent strength-ductility synergy, and the UTS, YS, and EL of the as-deposited (AD) specimens were 243.38 MPa, 141.81 MPa, and 22.15%, while the T6 heat-treated (T6) specimens achieved 334.71 MPa, 222.08 MPa, and 19.36%, respectively. Compared to the room-temperature mechanical properties, the T6 heat-treated 8% La₂O₃ samples at 250 ℃ maintain 66.21% in tensile strength, and the elongation increased by 4.81%. The enhanced strength-ductility synergy was achieved by incorporating the La elements during the WA-DED process, and the incorporation of La elements depended on a redox reaction: 3Mg+La₂O₃→2La+3MgO. Notably, the β₁ phases (segregated Nd and La) were observed at γ₁/Mg interfaces, underpinning a composite strengthening mechanism of the dislocation cutting mechanism and the dislocation bypass mechanism. These results provide a simple in-situ rare-earth reinforcement route to mitigate the strength-ductility trade-off in the Mg-RE alloys.