The effects of post heat treatment on the microstructure, aging kinetics, and room/elevated temperature mechanical properties of additively manufactured Inconel 718 superalloy were investigated. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD), as well as hardness, tensile, and creep testing were used for characterization. At temperatures higher than 1100 °C, homogenization treatment resulted in the appearance of equiaxed grains by recrystallization and diminishing the dislocation density. The precipitation activation energy for the homogenized and aged condition was obtained as 203.2 kJ/mol, which was higher than the value of ~160 kJ/mol for the as-built IN718 superalloy. Therefore, direct aging resulted in a faster aging response, which led to a significant improvement in tensile properties, as rationalized by the strengthening mechanisms. Direct aging treatment resulted in a higher elevated-temperature ultimate tensile strength (UTS) as well as the optimum creep life and the lowest minimum creep rate in comparison with other heat treatment routes, which were attributed to the presence of fine and uniformly dispersed strengthening precipitates in conjunction with the high dislocation density.