The influence of the sheet thickness ratio on the joint quality of clinched joints was investigated experimentally and numerically. AA6061-T6 sheets of 1.2 mm and 1.5 mm thickness were used to generate three sheet thickness ratios, 0.8, 1, and 1.25, and four different sheet thickness combinations, 1.2:1.5, 1.2:1.2, 1.5:1.5, 1.5:1.2. The cross-sections of the joints were investigated to study the joint geometry parameter such as neck thickness and undercut. In order to examine the static strength, energy absorption, and failure mechanism, lap shear tests were performed. In addition, a three-dimensional numerical model was developed using commercial FE software DEFORM-3D to conduct simulations. The results showed that the neck thickness of the joints rises as the sheet thickness ratio increases, while there was no clear trend observed in case of the undercut. Neck fracture was found to be the primary failure mode of the joints. Joints with larger sheet thickness ratio produced stronger joints with greater energy absorption. The numerical simulations had good correlation with the experimental results.