BACKGROUND: Digital self-monitoring tools, such as the experience sampling method (ESM), enable individuals to collect detailed information about their mental health and daily life context and may help guide and support person-centered mental health care. However, similar to many digital interventions, the ESM struggles to move from research to clinical integration. To guide the implementation of self-monitoring tools in mental health care, it is important to understand why and how clinicians and clients adopted, adapted, and incorporated these tools in practice. OBJECTIVE: Therefore, this study examined how clinicians and clients within a psychiatric center appropriated an ESM-based self-monitoring tool within their therapy. METHODS: Twelve clinicians and 24 clients participated in the piloting of the ESM tool, IMPROVE. After utilizing the tool, 7 clinicians and 11 clients took part in semistructured interviews. A thematic framework analysis was performed focusing on participants' prior knowledge and expectations, actual use in practice, and potential future use of ESM tools. RESULTS: Many participants experienced that the ESM tool provided useful information about clients' mental health, especially when clinicians and clients engaged in collaborative data interpretation. However, clinicians experienced several mismatches between system usability and their technical competencies, and many clients found it difficult to comply with the self-assessments. Importantly, most participants wanted to use digital self-monitoring tools in the future. CONCLUSIONS: Clinicians' and clients' choice to adopt and integrate self-monitoring tools in their practice seems to depend upon the perceived balance between the added benefits and the effort required to achieve them. Enhancing user support or redesigning ESM tools to reduce workload and data burden could help overcome implementation barriers. Future research should involve end users in the development of ESM self-monitoring tools for mental health care and further investigate the perspectives of nonadopters.