Supporting dementia carers is a global priority. As a Randomised Controlled Trial (RCT) (n = 352) of the Word Health Organization recommended, an internationally disseminated ‘iSupport’ e-health intervention was conducted, revealing no measurable benefits to the wellbeing of adult dementia carers. This process evaluation contributes original insights of the trial outcomes. Its aims were to ascertain the usability and acceptability of iSupport, participant engagement and adherence to iSupport, and contextual factors influencing its implementation and potential impact. The process evaluation followed a mixed-method design. The following data were collected from all participants randomised to iSupport (n = 175): (1) post-intervention evaluation questionnaire (n = 93) containing the 10-item System Usability Scale and bespoke items exploring acceptability, engagement, and perceived impact; (2) qualitative interviews (n = 52) with a sub-sample of participants who were purposively sampled according to age, scores on the outcome measures, and gender, as these interviews aimed to generate contextual detail and explanatory accounts; and (3) ‘Access’ data from the iSupport platform (n = 175). Descriptive statistics was used to report on the frequency of survey responses whilst a thematic analysis approach was followed to identify themes from the qualitative interview data. Data sets were analysed independently and then used with respect to one another in order to generate explanatory pathways related to the usability, acceptability, and the impact of iSupport. Despite good trial retention, 8.3% of participants (n = 32) did not spend any time on iSupport, and 54% (n = 94) spent between 30 min and 1.5 h. Factors driving this were the following: time constrains, method of delivery, and content characteristics. Positive impacts of iSupport were also described. Participants, including those with extensive caring experience, reported how iSupport had made them feel reassured, valued, and more able to ask for help. They also reported having an improved outlook on their caring role and on the needs and feelings of the person living with dementia. Research and practice should focus on exploring blended delivery, including self-directed and interactive components, such as regular contact with a health professional. These insights are critical for supporting the global implementation and adaptation of iSupport and offer valuable directions for future research.