Background <p>Measurement-based care (MBC) is the routine use of patient-reported outcome measures to inform treatment planning in mental health care. MBC provides clinicians with real-time indicators of clients’ symptoms and functioning, and organizations with aggregate outcome data to inform service evaluation. While our understanding of how to implement MBC is growing, there is very little evidence on how to sustain MBC within child and adolescent mental health services (CAMHS). The purpose of this project is to describe the conditions, and processes that promote sustainment of MBC in eight community child and adolescent mental health treatment programs.</p> Methods <p>We will use an explanatory, sequential mixed-methods design. Quantitative data on MBC sustainability will be derived from clinician surveys and the provincial electronic medical record. Following Yin’s multiple-case study methodology, qualitative data will be collected through interviews to explore factors that promoted sustainability across sites. We use the Dynamic Sustainability Framework as the theoretical model for this study and the Consolidated Framework for Implementation Research. The proposed project will take place in eight public CAMHS clinics from a single urban jurisdiction. Participants will include clinical staff from the sites and clients ages 11–17.</p> Discussion <p>This study will examine sustainment of MBC in CAMHS settings after external implementation support has been withdrawn. Knowledge of specific events, processes, and contextual conditions within clinics in this study that supported MBC sustainment will help practitioners implement and sustain MBC in CAMHS.</p> Trial registration <p>Not applicable.</p>