A nonuniform cement mantle significantly affects the long-term success of cemented total hip arthroplasty (THA). Over-insertion of the acetabular cup leads to an uneven layer of cement mantle, which can cause stress concentration and potential failure, such as implant loosening. This study investigates the effect of over-insertion of the acetabular cup on the stress distribution at the cement-cup and cement-bone interface after THA and compares with the non-over-inserted case. Finite element models of synthetic bone hemipelves implanted with over-inserted cemented cups with internal diameters of 28 and 36 mm were created from CT scan data. The models were validated with surface strains obtained from a composite hemipelvis implanted with a cemented all-polyethylene acetabular cup and used to simulate a one-leg standing activity of an average weight subject of 700 N. Over-insertion of the acetabular cup led to a thinner, nonuniform cement mantle, resulting in higher stresses in the superior and anterior periacetabular regions of the cortical bone and at the bone-cement and cement-cup interface, compared to the corresponding non-over-inserted case. At the bone-cement interface, our model predicted stress increases of up to 50% in the antero-superior and 35% in the postero-superior quadrant of the joint, an area where debonding frequently initiates. In addition, a larger head diameter was associated with lower rises in bone and cement stresses resulting from uneven cement mantles. Although stresses in the cement were below yield, raised stresses in this region are associated with crack formation and growth, which could lead to implant loosening.