For patients with mandibular bone defects, although reconstruction plates can be used for repair, achieving both occlusal function and facial aesthetics is challenging. In the present study, in vitro experiments and finite element analysis (FEA) were conducted to determine the biomechanical characteristics of multiple porous lattice structures of varying shapes and diameters that were used for mandibular implants. Additionally, an abutment designed to carry occlusal forces was added to the top of the implants. The stress distribution of four lattice designs (tetrahedron, quad-diametral-cross, hex-star, and hex-vase) of three sizes (2.5, 3.0, and 3.5 mm) in cubic porous models were analyzed by FEA. Subsequently, two optimal designs for 3D-printed titanium alloy were selected. These designs, featuring different lattice diameters (0.5, 0.7, and 0.9 mm), were tested to determine their elastic modulus, which was used in another FEA of a mandibular implant designed for a patient with a malignant tumor in the right mandible. This model, which included an abutment design, was subjected to a vertical force of 100 N and muscle forces generated by biting. This analysis was conducted to determine the elastic modulus of the implant and the values of stress and strain on the implant and surrounding bone. The lattice designs of quad-diametral-cross and hex-vase exhibited smaller high-stress regions than those of tetrahedron and hex-star. In vitro tests revealed that the elastic modulus of the lattices increased with the rod diameter. When these values were applied to mandibular implants, Young’s modulus decreased, which in turn increased the frictional stress observed at the interface between the abutment and the implant. However, the implant’s maximum stress remained below its yield strength (910 MPa), and the strain on the surrounding bone varied between 1500 and 3000 μstrain. As indicated by Frost’s theory, these implants are unlikely to damage the surrounding bone tissue and are likely to support bone growth.