Background: Caring for a family member with dementia puts caregivers at risk of overburdening. Electronic health (eHealth) support for caregivers offers an opportunity for accessible tailored interventions. The blended care self-management program "Partner in Balance" (PiB) for early-stage dementia caregivers was executed in Dutch dementia care organizations. The program combines face-to-face coaching with tailored Web-based modules. Next to an evaluation of program effectiveness, an evaluation of sampling and intervention quality is essential for the generalizability and interpretation of results.; Objective: The aim of this study was to describe the process evaluation from the perspective of both family caregivers (participants) and professionals delivering the intervention (coaches) to determine internal and external validity before the effect analysis and aid future implementation.; Methods: Implementation, sampling, and intervention quality were evaluated with quantitative and qualitative data from logistical research data, coach questionnaires (n=13), and interviews with coaches (n=10) and participants (n=49). Goal attainment scaling was used to measure treatment-induced change. Analyses were performed with descriptive statistics and deductive content analysis.; Results: The participation rate of eligible caregivers was 51.9% (80/154). Recruitment barriers were lack of computer and lack of need for support. Young age and employment were considered recruitment facilitators. All coaches attended training and supervision in blended care self-management. Deviations from the structured protocol were reported on intervention time, structure, and feedback. Coaches described an intensified relationship with the caregiver post intervention. Caregivers appreciated the tailored content and positive feedback. The blended structure increased their openness. The discussion forum was appreciated less. Overall, personal goals were attained after the program (T>50). Implementation barriers included lack of financing, time, and deviating target population.; Conclusions: Participants and coaches were satisfied with the intervention, but adapting the content to specific subgroups, for example, younger caregivers, was recommended. Implementation of the program requires more awareness of the benefits of blended care self-management programs and training in tailored self-management skills.; Trial Registration: Dutch Trial Register (NTR): NTR4748; http://www.trialregister.nl (Archived by WebCite at http://www.webcitation.org/6vSb2t9Mg).; ©Lizzy MM Boots, Marjolein E de Vugt, Claudia MJ Smeets, Gertrudis IJM Kempen, Frans RJ Verhey. Originally published in the Journal of Medical Internet Research (http://www.jmir.org), 19.12.2017.