Cardiovascular diseases and type 2 diabetes mellitus are highly prevalent chronic diseases. They have similar risk factors and are preventable with lifestyle changes. Mobile applications (apps) can provide behaviour change interventions widely at a relatively low cost. This thesis aimed to identify risk assessment tools suitable for laypeople to determine their cardiovascular and diabetes risk, develop an app for risk awareness and prevention of the two diseases, and test the feasibility of the mobile health intervention. Chapter 2 contains a rapid review of cardiovascular and diabetes risk models. In Chapters 3 and 4, two models based on lifestyle-related risk factors were externally validated in a large Australian cohort. In Chapter 5, the literature was systematically reviewed to evaluate the effectiveness of preventive mobile interventions for the two diseases. Chapter 6 comprises a cross-sectional analysis of mobile health use in older Australians. Chapter 7 describes the development process of a smartphone app according to a framework and iterative testing of the app's usability in a small sample. In Chapter 8, the feasibility of the intervention was assessed over three months with app data and a survey. The rapid review in Chapter 2 included the risk models routinely used in Australian clinical practice and two lifestyle-based risk models without Australian validation. In Chapters 3 and 4, the lifestyle-based diabetes risk model performed satisfactorily, but the cardiovascular model predicted a 5-year risk poorly. The systematic review in Chapter 5 found some indications for the effectiveness of mobile interventions in preventing cardiovascular disease and diabetes. The cross-sectional analysis in Chapter 6 showed low mobile health use in older Australians. The app developed in Chapter 7 encompassed four modules: risk assessment, goal setting, self-monitoring, and health information. Usability testing participants found the app easy to use. In the feasibility study in Chapter 8, 20 out of 46 participants never used the app, 15 dropped out, and 8 used the app weekly. The overall app quality rating was satisfactory. Although the intervention was easy to use, the levels of adoption and sustained use were low. Further research is needed to adapt the app to make it more appealing and provide greater benefits. The potential utility of digital health interventions is vast, but they still need to live up to their expectations.