BackgroundLockdown during the pandemic has had significant impacts on public mental health. Previous studies suggest an increase in self-harm and suicide in children and adolescents. There has been little research on the roles of stringent lockdown.AimsTo investigate the mediating and predictive roles of lockdown policy stringency measures in self-harm and emergency psychiatric presentations.MethodThis was a retrospective cohort study. We analysed data of 2073 psychiatric emergency presentations of children and adolescents from 23 hospital catchment areas in ten countries, in March to April 2019 and 2020.ResultsLockdown measure stringency mediated the reduction in psychiatric emergency presentations (incidence rate ratio of the natural indirect effect [IRR^NIE] = 0.41, 95% CI [0.35, 0.48]) and self-harm presentations (IRR^NIE = 0.49, 95% CI [0.39, 0.60]) in 2020 compared with 2019. Self-harm presentations among male and looked after children were likely to increase in parallel with lockdown stringency. Self-harm presentations precipitated by social isolation increased with stringency, whereas school pressure and rows with a friend became less likely precipitants. Children from more deprived neighbourhoods were less likely to present to emergency departments when lockdown became more stringent,ConclusionsLockdown may produce differential effects among children and adolescents who self-harm. Development in community or remote mental health services is crucial to offset potential barriers to access to emergency psychiatric care, especially for the most deprived youths. Governments should aim to reduce unnecessary fear of help-seeking and keep lockdown as short as possible. Underlying mediation mechanisms of stringent measures and potential psychosocial inequalities warrant further research.