Promoting youth mental health is a critical public health priority and merits robust policy and practice responses, inclusive of youth-centred and upstream interventions that address the root factors contributing to mental health outcomes. To that end, non-familial youth-adult relationships can powerfully impact youths’ healthy development, mental health and well-being, and capacities for enacting change within home, school, and policy contexts. Agenda Gap is a youth mental health promotion programme based on this principle, in which adult facilitators support youth (aged 15–24) in co-exploring activities focused on mental health promotion and policy advocacy, while fostering supportive, trusting intergenerational relationships. This study presents a realist evaluation of Agenda Gap, drawing on realist qualitative interviews with youth participants (n = 18) and adult collaborators (n = 4). We constructed 15 initial programme theories in four theoretical areas that we subsequently explored via analysis of the realist interview data. The analysis used the middle-range theories of ‘Third Space’ and ‘Third Place’ to conceptualize and articulate how causal mechanisms were produced from the non-familial youth-adult relationships that are foundational to the programme. Results are presented across 10 context-mechanism-outcome (CMO) configurations in three sections: (1) Agenda Gap Facilitation Activates ‘Third-Space/Third-Place’ Mechanisms; (2) Youth Enhance Personal Mental Health while also Supporting Mental Health of Friends and Family; (3) Youth Become Inspired and Informed to Act as Advocates and Policy Change Agents. Taken together, these findings provide insights into the important mechanisms of non-familial intergenerational trust building and demonstrate how a strengths-based conceptualization of youth mental health supports mental health maintenance, promotion, and advocacy for this population.