Background: A working alliance (WA) is a multidimensional construct signifying a collaborative relationship between a client and therapist. Systematic reviews of therapies to treat depression and anxiety, almost exclusively in adults, demonstrate that WA is essential across psychotherapies. A functional WA predicts greater uptake of interventions, client engagement, adherence to treatment, and symptoms reduction. Conversely, ruptured and/or low WA reduces the effectiveness of known-efficacious treatments. However, there are critical gaps in our understanding of the importance of WA in low-intensity therapies for young people with depression and anxiety. We sought to understand the following research questions in young persons aged 14-24 years: 1) the role of working alliance in the ongoing management of anxiety and depression? 2) how do the different working alliance elements influence treatment outcomes?3) the client/therapist characteristics deemed essential in forming and maintaining a working alliance? Methods: We conducted a scoping review to summarise and synthesise the findings collaboratively with young people with lived experiences of anxiety/depression. Then, we conducted key stakeholders' consultations with clinicians [lay health counsellors (n=6), clinical psychologists (n=2), occupational therapists (n=2), and psychiatrists (n=2)], and young people with lived experiences (n=20). Finally, we convened workshops to triangulate/synthesise findings from the scoping review and stakeholder consultations to develop the mechanistic framework, hypothesising pathways by which WA may influence treatment outcomes. Results: We analysed 27 studies; most were done in high-income countries. Cognitive behavioural therapy was the most common treatment modality (16/27). Most therapy sessions were conducted one-to-one (18/27), with only two (2/27) studies utilising digital therapy platforms. A functional working alliance was recognised as an active ingredient in psychotherapies targeting anxiety and depression in young persons aged 14-24 years. Previous research has theorised that improvements in the working alliance are associated with improvements in relationships, self-esteem, positive coping strategies, optimism, adherence to treatments, and emotional regulation. During the stakeholders’ consultations, young people with lived experience expressed that; a favourable therapy environment, regular meetings, collaborative goal setting and confidentiality were vital in forming and maintaining a functional WA. For a clinician, ssetting boundaries, maintaining confidentiality, excellent communication skills, and being non-judgmental and empathic were considered important for facilitating a functional WA.Conclusions: Although more research is needed to understand WA's influence in managing anxiety and depression in young people, we recommend routine evaluation of WA. Furthermore, there is an urgent need to identify strategies that promote WA in psychotherapies to optimise the treatment of anxiety and depression in young people.