Additive friction stir deposition (AFSD) is a solid-state additive manufacturing process that enables the fabrication of fully dense metallic components without common fusion-related defects. Inconel 718, widely used in aerospace and energy sectors, requires high structural reliability; therefore, evaluating its response to AFSD is essential for advanced applications. This study investigates the effects of AFSD on IN718 by comparing the mechanical properties and microstructure of the as-deposited material with the feedstock condition. Tensile testing showed that the ultimate tensile strength (UTS) increased by 5% along the traverse direction, whereas elongation was reduced compared to the feedstock. In contrast, build-direction tensile specimens exhibited lower UTS and substantially reduced elongation, revealing mechanical anisotropy. Microhardness increased by 20%, consistent with substantial grain refinement from 11 µm to 3 µm due to dynamic recrystallization during deposition. X-ray diffraction (XRD) revealed no clearly detectable secondary phase formation after AFSD within the resolution limits of conventional XRD, suggesting that the increased hardness and traverse-direction strength can be partly explained by grain refinement. Elemental mapping detected oxygen-enriched Al/Ti regions at interlayer boundaries, which may contribute to the reduced build-direction ductility. Overall, AFSD refined the microstructure, enhanced hardness, and improved traverse-direction strength, while build-direction tensile testing revealed anisotropic mechanical behavior.