This study investigates the relationship between the microstructure and the mechanical properties of additively manufactured (AM) 18Ni(300) Maraging Steel undergone post-processing heat treatment. As-built additively manufactured 18Ni(300) Maraging steel was subjected to a heat treatment, solutionizing, in order to determine the effect of the post-processing heat treatment on microstructural evolution. X-ray diffraction (XRD), optical and scanning electron microscopy (OM / SEM), as well as Scanning Transmission Electron Microscopy (STEM), were used for in-depth materials characterization. Resultant changes in mechanical properties determined via hardness testing were linked with the variation of microstructural features (primary precipitate formation, size, and distribution) as well as process-induced porosity levels. In addition, it was found that sample size and gas flow of argon (Ar) play a critical role in determining the as-built microstructure.