Youth homelessness in the United Kingdom is a complex, multifaceted issue encompassing numerous determinants, pathways, and often deleterious effects on mental health and wellbeing. Despite the disproportionately high prevalence of mental disorders amongst this population, few access or receive appropriate treatment, predominantly due to limited availability, lack of awareness, and fear of stigmatisation. Evidently, there is an urgent need to address the health inequalities experienced by young people experiencing homelessness (YPEH), through implementing novel strategies which encourage uptake, engagement, and adherence to effective treatments. This Thesis explored the potential for physical activity (PA) as a viable means by which YPEH may be more likely to engage with psychological therapy. While this concept initially underpinned the primary aim of this project, a comprehensive review of the evidence-base indicated that psychologically-informed PA could offer a standalone treatment option- with comparable effects to psychological therapies. Utilising these findings, and evidence derived from the extant literature, a theoretically-informed intervention comprising PA and Dialectical Behavioural Therapy Skills Training (DBT-ST) was developed for YPEH, to establish feasibility, acceptability, and appropriateness of implementing this complex intervention with this population. While findings from this Pilot Study were evaluated per protocol, this period coincided with the onset of the global COVID-19 pandemic, which presented unprecedented challenges, and requirements to adapt this research project accordingly. Paradoxically, these extraordinary circumstances afforded a unique insight into the effects of isolation and restrictions on the well-being, self-esteem, and PA behaviours of relatively large sample (n = 50) of YPEH; with findings elucidating the need to incorporate sedentary behaviour (SB) as a distinct variable to PA, for the ensuing multi-site study. A two-arm study comparing psychologically-informed group PA delivered either alone, or with adjunctive DBT-ST was designed, implemented, and evaluated with two groups of YPEH. Despite several implementation challenges, mixed-method evaluations of the interventions provided several important contributions to the field, including the effectiveness of groupbased PA, barriers and facilitators to implementation, and postulated mechanisms of effect for supporting positive outcomes with this population. The accumulated evidence acquired throughout this Thesis provides novel, timely, and important insights into not only 'what works' when supporting YPEH, but also 'how, and why' opportunities can engender positive effects.