A WE43 (Mg-4Y-3Nd-0.5Zr, wt.%) magnesium rare-earth alloy thin-wall component was fabricated via wire arc additive manufacturing, and its microstructure and mechanical properties were comprehensively investigated through multiscale characterization, microhardness, and tensile tests. The influence of direct aging (T5) and solid solution + aging (T6) on the microstructure evolution and mechanical properties were comparatively studied. Results indicate that the deposited WE43 alloy has a uniform equiaxed crystal matrix, with an average grain size of 25.3 μm. Reticulated eutectic structure (α-Mg+Mg41Nd5/Mg24Y5) was formed due to Nd and Y element liquid segregation at grain boundaries. The alloy’s tensile strength in deposited state reaches 190 MPa. Post T5 aging imparts a peak hardness increase from 74 HV0.2 to 91 HV0.2, with persistence of significant eutectic structure. T6 treatment yields a peak ageing hardness (108 HV0.2), with the eutectic structure dissolve completely while a small amount of Mg24Y5 remained in matrix. T6 treatment enhances the alloy’s tensile strength to 283 MPa, but it also induces significant grain growth and reduces the elongation in vertical direction more obviously than that in horizontal direction.