This systematic review aimed to identify mechanisms of psychological change following exposure to nature within an adolescent population. Keyword searches within Scopus, PsychINFO and Web of Science were carried out to include articles that were published by 14th September, 2021. Records were reviewed in line with inclusion criteria: adolescents with an average age of 24 and under, inclusion of exposure to nature compared to a control condition using an experimental or quasi-experimental design and outcomes relating to mental health and psychological status. The systematic review resulted in the inclusion of 27 papers that were assessed for methodological quality and manually searched for the inclusion of mediation analyses. A range of psychological outcomes were identified and grouped into 10 categories: Mood and Affect, Mental Health, Wellbeing, Perceived Restoration, Stress, Energy, Cognitive Functioning, Resilience, Self-Concept and Pro-Social Behaviour. Only one formal mediation analysis was reported, highlighting a mediating role of belonging in increases in resilience. Limitations include the majority use of undergraduate samples and over half of the papers being of low methodological quality. No firm conclusions on key mechanisms in an adolescent population were made due to insufficient evidence of mediating variables. The development of methodologically rigorous experimental studies with the inclusion of statistical pathway modelling is needed to test and specify plausible mechanisms. Adolescence is a significant period of vulnerability for the development of depression, highlighting it as a critical time for preventative interventions. Nature contact is a modifiable aspect of lifestyle that has shown promise for the prevention of mental ill health and requires further exploration in an adolescent population. 103 adolescents with depressive symptomology were randomised either to an interactive, online, nature-based psychoeducation session or to a wait-list control. Pre/post measures and a daily diary were completed over a two-week period, with intervention participants recommended to increase their nature contact during this time. Depression, anxiety, negative affect and perceived stress significantly decreased over the two-week period in both conditions. Nature connection, mindfulness, wellbeing and positive affect also significantly increased over this time in both conditions. No significant differences were found on outcomes between those who received the intervention and control participants. Potential reasons for a lack of group differences in outcomes are discussed. Across the two weeks, both conditions had contact with nature at a level that may have been sufficient to improve outcomes for all, supporting a potential beneficial effect of nature contact. Interventions supporting increased nature contact in an adolescent population warrant further research.