Background: The MedFit application is designed to facilitate people with cardiovascular disease (CVD) to participate in an exercise-based rehabilitation programme remotely. This paper details development of the formative research process outlined by the Medical Research Council. Purpose: To describe the development of the MedFit app, by following the early stages of the formative research process; development and feasibility/piloting. Methods: Following the creation of the first prototype of the app, the feasibility and acceptability of the prototype application was tested in focus groups. The focus group script was developed using a questionnaire (N=119 MedEx participants; 64.7% male; mean age 65 ± 8.86 years) based on usability theory (UTAUT2). Twenty-six cardiac rehabilitation participants took part in the five focus groups (65% male; mean age 64±8.2 years) to provide feedback on the prototype app. Focus groups were transcribed verbatim and in-depth content analysis was performed. Results: The results of the questionnaire revealed that performance expectancy, social influence, hedonic motivation, behavioural intention, effort expectancy and facilitating conditions all rated highly among respondents. These constructs were used to develop the focus group script. Following in-depth content analysis, four main themes were identified; support, app as a mentor/guide, translation of activity from gym to home and technology knowledge gap. Conclusion: The formative research process of the app development was undertaken to develop the MedFit app. This work will provide guidance for future research by incorporating a best practice framework for mHealth intervention development and a user-centered design approach.