Background: Personal health records (PHRs) provide the opportunity for self-management support, enhancing communication between patients and caregivers, and maintaining and/or improving the quality of chronic disease management. Their implementation is a multi-level and complex process, requiring a holistic approach that takes into account the technology, its users and the context of implementation. The aim of this research is to evaluate the fidelity of a PHR in chronic care (the degree to which it was implemented as intended) in order to explain the found effects. Methods: A convergent parallel mixed methods design was used, where qualitative and quantitative data were collected in parallel, analyzed separately, and finally merged. Log data of 536 users were used to gain insight into the actual long-term use of the PHR (the dose). Focus group meetings among caregivers (n = 13) were conducted to assess program differentiation (or intended use). Interviews with caregivers (n = 28) and usability tests with potential end-users (n = 13) of the PHR were used to understand the responsiveness and the differences and similarities between the intended and actual use of the PHR. Results: The results of the focus groups showed that services for coaching are strongly associated with monitoring health values and education. However, the PHR was not used that way during the study period. In the interviews, caregivers indicated that they were ignorant on how to deploy the PHR in current working routines. Therefore, they find it difficult to motivate their patients in using the PHR. Participants in the usability study indicate that they would value a PHR in the future, given that the usability will be improved and that the caregivers will use it in daily practice as well. Conclusions: In this study, actual use of the PHRs by patients was influenced by the responsiveness of caregivers. This responsiveness is likely to be strongly influenced by the perceived support when defining the differentiation and delivery of the PHR. A mixed-methods approach to understand intervention fidelity was of added value in providing explanations for the found effects that could not be revealed by solely focusing on the effectiveness of the technology in an experimental trial.