mHealth systems can be used for patients in their diabetes self-management, but usability evaluations are often needed to determine how to make them more useful for the diabetes patient user in the monitoring and managing of their disease. Activity Theory (AT) was developed within Russian psychology to define the work and activity process in an activity system. AT was here considered to also be a particularly suitable framework for inspiration in usability evaluation both for the whole evaluation process and also for the usability task determination in this process for diabetes patient users. In the following paper, examples are provided from four usability studies using both user-based and expert usability methods in evaluation showing how AT was applied to guide the thoughts in evaluating the usability of two mHealth self-management systems for diabetes. Experiences and insights are provided from this process.