Objectives: Caring for a person with dementia can be challenging over the years. To support family carers throughout their entire caregiving career, interventions with a sustained effectivity are needed. A novel 6-week mobile health (mHealth) intervention using the experience sampling method (ESM) showed positive effects on carers’ well-being over a period of 2 months after the intervention. In this study, the effects after 6 months of the selfsame intervention were examined to evaluate the sustainability of positive intervention effects. Method: The 6-week mHealth intervention consisted of an experimental group (ESM self-monitoring and personalized feedback), a pseudo-experimental group (ESM self-monitoring without feedback), and a control group (providing regular care without ESM self-monitoring or feedback). Carers’ sense of competence, mastery, and psychological complaints (depression, anxiety and perceived stress) were evaluated pre- and post-intervention as well as at two follow-up time points. The present study focuses on the 6-month follow-up data (n = 50). Results: Positive intervention effects on sense of competence, perceived stress, and depressive symptoms were not sustained over 6-month follow-up. Conclusion: The benefits of this mHealth intervention for carers of people living with dementia were not sustained over a long time. Similarly, other psychosocial interventions for carers of people with dementia rarely reported long-lasting effects. In order to sustainably contribute to carers’ well-being, researchers and clinicians should continuously ensure flexible adjustment of the intervention and consider additional features such as ad-hoc counseling options and booster sessions. In this regard, mHealth interventions can offer ideally suited and unique opportunities.