Modularization is a strategy used for handling the demand for external complexity with less internal complexity, which leads to higher profits and more efficient product development processes. However, modularity is often driven in silos, not crossing into the engineering fields of mechanics, electronics, and software. Therefore, we present the MESA (Mechanics, Electronics, and Software Architecture) tool—a tool that can be used to visualize modular product architectures across mechanics, electronics, and software. The tool demonstrates how a change in one domain affects the rest and how well aligned the modularity in the different domains is. The tool has been tested in two case companies that were used for case application and has helped provide information for making key design decisions in the development of new product families.