Zinc-based alloys are emerging as the next generation of degradable metals due to their favorable degradation rates. Additive manufacturing offers the capability to produce personalized structures, making it a promising method for generating biomedical metal components. However, during laser powder bed fusion (LPBF), zinc-based alloys can experience significant evaporation because of their low melting point, which often results in poor surface quality. Optimizing powder morphology and particle size presents a potential strategy to enhance forming quality. In this study, two Zn-1Mn-0.4Mg (mass fraction, %) alloy powders with particle sizes of 15–53 μm and 30–75 μm were produced using electrode induction gas atomization (EIGA) and the plasma rotating electrode process (PREP), respectively. Bulk alloys were fabricated using LPBF, and the processing windows for both powders were compared to investigate the effects of powder morphology and particle size on the microstructural and mechanical properties. The results indicated that PREP powders, which exhibited better sphericity and larger particle size, had a broader and more stable LPBF processing window. This led to bulk samples with higher density and smaller grain sizes compared to those prepared with EIGA powders. Under optimal processing parameters (P = 50 W, V = 600 mm/s), both bulk sample types displayed similar tensile yield strength (210 MPa) and ultimate tensile strength (285 MPa). However, the compressive yield strength of the EIGA + LPBF samples ((288.2 ± 4.3) MPa) was slightly higher than that of the PREP + LPBF samples ((258.5 ± 3.5) MPa). By contrast, the PREP + LPBF samples demonstrated significantly greater tensile elongation (21.8% ± 2.3%) compared to the EIGA + LPBF samples (3.3% ± 0.7%). This enhanced ductility was primarily attributed to the more homogeneous thermal conductivity and strain distribution in PREP powders during the LPBF process. These findings offer valuable insights into raw material selection, microstructural control, and mechanical property optimization for LPBF-processed low-melting-point zinc-based alloys.