Adhesive joints are wildly used in various engineering applications such as aerospace and aviation. However, these joints are often susceptible to adhesion defects, which can compromise their mechanical performance. This paper presents a comprehensive numerical investigation into the effects of adhesion defects on the mechanical properties of single lap joints (SLJs). A two-dimensional finite element model, incorporating contact elements and cohesive zone material in the adhesive, is established. Various types of defects, including through-thickness, surface, and embedded defects, are considered, and their distributions and sizes are discussed. To validate the model, a comparison with existing literature results is performed, followed by a parametric study. The results indicate that the presence of adhesion defects leads to a reduction in the strength and stiffness of SLJs. Furthermore, the overall strength decreases with increasing defect size, and the types and distributions of defects influence the failure modes of the joints. Finally, quadratic polynomial fitting functions are proposed to evaluate the peak loads of SLJs with adhesive defects.