Background: Physical activity determinant studies now often include both environmental and sociocognitive factors but few of them acknowledge and explore the mechanisms underlying relevant environmental influences. This study explored environmental correlates of children's self-reported physical activity and potential mediation through the Theory of Planned Behavior (TPB) and habit strength. Methods: Six hundred and twenty-one pupils aged 9-11 years were recruited from 4 primary schools in the UK. TPB variables, habit strength and environmental variables were assessed at baseline. Self-reported physical activity was assessed 1 week later. Results: Mediation tests revealed that 43% of the association between convenient facilities and intention was mediated through subjective norms (17%) and habit (26%), while 15% of the association between convenient facilities and physical activity was mediated through habit strength alone. A significant direct effect of convenient facilities and resources in the home environment on physical activity was also found. The school environment was not significantly related to children's physical activity intentions or behavior. Conclusion: The results suggest that the environment influences children's physical activity both directly and indirectly and that habit strength seems to be the most important mediator for this association.