Multiple sclerosis (MS) is a chronic neurological condition affecting around 2.2 million people worldwide. The illness includes a range of symptoms, with fatigue considered to be one of the most disabling. This paper describes how a pragmatic and iterative approach, supported by usability and resonance testing, was used to build a minimum viable product of MS Energize-or MS Energise in UK English regions. MS Energise is a mobile application focused on self-management of fatigue for people with MS. The iterative approach included various stages of testing, during which user feedback including comments about interface, navigation and content, was sought to inform incremental app development and continual improvement. Usability testing was conducted with 11 people with longstanding multiple sclerosis in New Zealand and the United Kingdom, and focused on particular sections of the app as well as the accessibility of the app to users with MS. Two participants contributed to further resonance testing post-release to ensure the app was perceived as relevant and useful to the user. The usability testing and resonance testing phases suggested that user experience of MS Energise was mostly positive. Participants provided a number of suggestions for improvements to aspects of content and design; some of which we implemented during our app development process. Findings will also contribute to future planning and design iteration to enhance the user experience. The next step is further improvement of MS Energise prior to a trial of its clinical and cost effectiveness.