CONTEXT: Physical activity can improve cognitive and mental health, but the underlying mechanisms have not been established. OBJECTIVE: To present a conceptual model explaining the mechanisms for the effect of physical activity on cognitive and mental health in young people and to conduct a systematic review of the evidence. DATA SOURCES: Six electronic databases (PubMed, PsycINFO, SCOPUS, Ovid Medline, SportDiscus, and Embase) were used. STUDY SELECTION: School-, home-, or community-based physical activity intervention or laboratory-based exercise interventions were assessed. Studies were eligible if they reported statistical analyses of changes in the following: (1) cognition or mental health; and (2) neurobiological, psychosocial, and behavioral mechanisms. DATA EXTRACTION: Data relating to methods, assessment period, participant characteristics, intervention type, setting, and facilitator/delivery were extracted. RESULTS: Twenty-five articles reporting results from 22 studies were included. Mechanisms studied were neurobiological (6 studies), psychosocial (18 studies), and behavioral (2 studies). Significant changes in at least 1 potential neurobiological mechanism were reported in 5 studies, and significant effects for at least 1 cognitive outcome were also found in 5 studies. One of 2 studies reported a significant effect for self-regulation, but neither study reported a significant impact on mental health. LIMITATIONS: Small number of studies and high levels of study heterogeneity. CONCLUSIONS: The strongest evidence was found for improvements in physical self-perceptions, which accompanied enhanced self-esteem in the majority of studies measuring these outcomes. Few studies examined neurobiological and behavioral mechanisms, and we were unable to draw conclusions regarding their role in enhancing cognitive and mental health.