BACKGROUND: There is now widespread recognition within adolescence mental health research of the ethical imperative and benefits of coproduction. This has led to the development of best practice guidelines and the routine reporting of coproduction methods. However, there are unique considerations associated with involving young people in research with varying designs, e.g., longitudinal. Ideally, these decisions are made in consultation with young people themselves. Thus, the objective and novel contribution of this paper is (1) the involvement of young people in the development of the MHIM protocol, including an evaluative framework, and (2) a study protcol that utilises coproduction in the context of a longitudinal mental health project. METHODS: We present a coproduction protocol (MHIM-YPAG), developed with young people (pre-study YPAG), that describes the planned approach to involving young people in the MHIM study. MHIM will use an accelerated longitudinal-cohort design that will combine ecological momentary assessment, bio-sampling, radar-based sleep measurement and online questionnaires to examine the relationship between daily-life-experiences and adolescence mental health. The protocol describes how young people will be involved in MHIM and how impact will be evaluated. Within this context, coproduction relates to (1) the codevelopment, with young people, of the study's structure and (2) their involvement as researchers, e.g., recruitment, data interpretation and knowledge-exchange. PATIENT OR PUBLIC CONTRIBUTION: As one of the first protocols of its kind, our coproduction protocol, informed by literature and consultations with young people (pre-study YPAG), can provide a template for planning and reporting coproduction in mental health studies of adolescence.