The laser powder bed fusion (LPBF) 3D printing technology opens up new opportunities for additive manufacturing of heat-resistance superalloys. At the same time, the LPBF method needs to be improved for critical applications. In addition, post-processing techniques for the LPBF-built Inconel alloys should be developed and integrated. This work focuses on observing the structural and strengthening mechanisms in the LPBF-produced Inconel 718 alloy subjected to thermomechanical post-processing. Various heat treatments combined with a hot isostatic pressing (HIP) were applied to provide the required grain-size microstructure and phase state, increasing the hardness and material density. Furthermore, the surface layers of the heat-treated samples were finished and hardened by an ultrasonic impact treatment (UIT) to form the grain-size nanostructure and compressive residual macro-stresses in the near-surface layers. The yield strength, hardening intensity, and local plasticity magnitudes were theoretically evaluated for the LPBF-printed and thermomechanically post-processed Inconel 718 superalloy, considering the microhardness and structure/phase/chemical composition using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis. The outcome of this study shows the formation of a pore-free microstructure with a more homogeneous distribution of Nb in solid solution, \(\gamma ^{\prime\prime}\) precipitation, and MC carbides after HIP combined with heat treatments, increasing the hardening intensity by 35–38%. UIT severe plastic deformation increased the subsurface hardening intensity by 58–68% of the heat-treated 718 alloy samples due to the grain refinement and increase in dislocation density.