In the automotive industry, spot-welding is the most common method of joining components. Thus, determining the service life of spot-welds is of great importance in designing assemblies or structures. It is well-known that lately there has been a trend in the industry toward reducing the fuel consumption and harmful gasses emissions, as well as the weight of structures with the application of the lightweight materials, like aluminum alloys. In this paper, research is presented on the behavior of a spot-weld between the plates made of the two dissimilar materials—aluminum alloy and steel. In addition, the influence of the plates’ thickness and the weld nugget’s diameter on welds’ service life is presented. In this analysis, a concept of the interface crack between the two linear elastic materials was applied. Obtained results show that the plates’ thickness and the nugget’s size, as well as the working load, impose significant influences on the service life of a spot-weld between the two dissimilar materials.