To improve the processability and mechanical properties of the selective laser melting (SLM) low Sc content Al-Mg-Sc-Zr alloy, Mn was used to partially replace Mg. The processability, microstructure, and mechanical properties of the SLM-fabricated Al-Mg-Mn-Sc-Zr alloy were systematically investigated by density measurement, microstructure characterization, and tensile testing. The results revealed that dense samples could be obtained by adjusting the SLM process parameters. The alloy exhibited a fine equiaxed-columnar bimodal grain microstructure. The presence of primary Al₃Sc and α-Al(Mn,Fe)Si particles contributed to the grain refinement of the alloy with an average grain size of 4.63 μm. Upon aging treatment at 350 °C for 2 h, the strength and elongation of the alloy were simultaneously improved due to the precipitation of Al₃Sc nanoparticles and the formation of the 9R phase. This study demonstrates that the strength-plasticity trade-off of the aluminum alloy can be overcome by utilizing SLM technology and subsequent post-heat treatment to induce the formation of the long-period stacked ordered phase.