<p>The advantages of ultralight Mg-Li alloy in manufacturing fields such as aerospace and vehicle transportation render it highly valuable for applications. However, the traditional additive manufacturing methods based on melting and solidification are prone to inducing internal defects during the fabrication of Mg-Li alloy, making it unsuitable for application in principle. In this work, friction stir additive manufacturing (FSAM) was successfully applied to a duplex Mg-9Li-3Al-Zn alloy (LAZ931). The microstructure, mechanical properties, and corrosion resistance of the LAZ931 alloy after FSAM were investigated. After the process, both the α-Mg phase and the β-Li phase experienced dynamic recrystallization, accompanied by the precipitation of fine MgLi₂Al and AlLi particles. The microhardness is distributed regularly with the distance between passes and is significantly improved compared with the base material. The strength of the LAZ931 alloy after FSAM was enhanced by combining grain refinement and solution strengthening. Concurrently, the corrosion current density of alloys after FSAM was reduced from 57.8 to 15.4&#xa0;μA cm⁻². The FSAMed LAZ931 alloy exhibited the synergistic enhancement of strength, ductility, and corrosion resistance, which indicated that FSAM has the potential to broaden the application of Mg-Li alloys.</p>