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Background: The rising prevalence of adolescent mild depression in the UK and the paucity of evidence-based interventions in non-specialist sectors where most cases present, creates an urgent need for early psychological interventions. Randomised controlled trials (RCTs) are considered the gold standard for obtaining unbiased estimates of intervention effectiveness. However, the complexity of mental health settings poses great challenges for effectiveness evaluations. This paper reports learning from an embedded process evaluation of the ICALM RCT which tested the feasibility of delivering Interpersonal Counselling for Adolescents (IPC-A) plus Treatment as Usual (TAU) versus TAU only for adolescent (age 12–18) mild depression by non-qualified mental health professionals in non-specialist sectors. Methods: A qualitative mixed methods process evaluation, drawing on Bronfenbrenner's socioecological model to investigate key influences on trial delivery across macro-(e.g. policy), meso-(e.g. service characteristics) and micro-(e.g. on-site trial processes) contextual levels. Data collection methods included 9 site questionnaires, 4 observations of team meetings, policy documents, and 18 interviews with stakeholders including therapists, heads of service and managers. Thematic analysis focused on understanding how contextual features shaped trial implementation. Results: The ICALM trial concluded in 2022 having only randomised 14 out of the target 60 young people. At a macro-level, trial delivery was impacted by the COVID-19 pandemic, with services reporting a sharp increase in cases of (social) anxiety over low mood, and backlogs at central referral points which prolonged waiting times for mild cases (e.g. low mood). An interaction between high demand and lack of capacity at a meso-service level led to low prioritisation of trial activities at a micro-level. Unfamiliarity with research processes (e.g. randomisation) and variation in TAU support also accentuated the complexities of conducting an RCT in this setting. Conclusions: Conducting a RCT of IPC-A in non-specialist services is not feasible in the current context. Failure to conduct effectiveness trials in this setting has clinical implications, potentially resulting in escalation of mild mental health problems. Research done in this setting should adopt pragmatic and innovative recruitment and engagement approaches (e.g. creating new referral pathways) and consider alternative trial designs, e.g. cluster, stepped-wedge or non-controlled studies using complex systems approaches to embrace contextual complexity. Trial registration: ISRCTN registry, ISRCTN82180413. Registered on 31 December 2019. [ABSTRACT FROM AUTHOR]