The promises of mobile technology in healthcare have led to a great many mobile apps in public app stores that target patients with specific illnesses. Medical experts have criticized the status quo of mobile medical apps owing to the low level of professional medical involvement in mobile app design, leading to weak clinical performance and a poor integration of these tools into clinical practice. Grounded in an action design research study, we build and evaluate a mobile app for elderly patients with age-related macular degeneration. We formalize our learnings and provide a set of design principles to guide the effective and feasible construction of mobile medical apps. Our study systematically develops design knowledge that helps to bridge the current gap between the rapid advances in mobile technology and the specific needs of the healthcare sector.