The mechanical characteristics, phase composition, residual macroscopic stresses, and surface topography of Co–Cr–Mo–W alloy fabricated using the additive (selective laser melting of powder—SLM) and casting (CT) technologies and modified by ultrasonic impact treatment (UIT) are investigated. A single-contact normal impact-loading mode of UIT is employed. As demonstrated, the macrodefects, including defects of incomplete melting, pores, significant surface roughness, and high levels of residual tensile stresses, which are inherent consequences of 3$D$ printing, are effectively eliminated by short-term UIT in an inert environment. A correlation between the mechanical properties of modified surface layers of the SLM and CT samples and their structural–phase state after UIT exposure for 50 s is established. The strengthening effect of the surface layer of the additively manufactured Co–Cr–Mo–W alloy (by 2 times) is due to the formation of compressive stresses of the first kind (-600 MPa) and martensitic transformation. The Co–Cr–Mo–W alloy obtained by casting technology has lower hardness both in the initial state and after UIT. The strengthening effect does not exceed 1.7 times and is achieved due to a higher level of compressive stresses (-900 MPa) and carbide component refinement.