Background. The prevalence of mental ill health increases with age through childhood and adolescence and likely impairs mental and physical health in adulthood. Little is known about the combined influence of sleep, sedentary time (ST), and physical activity on youth mental health. This study examined associations between youth 24-hour activity behaviour compositions and mental health indicators, and investigated predicted differences in mental health when time was reallocated between activity behaviours.Methods. Demographic information and anthropometric data were collected from 359 participants (aged 9-13 years; 50.7% girls). 24-hour activity behaviours (sleep, ST, light physical activity (LPA) and moderate-to-vigorous physical activity (MVPA)) were assessed using a wrist accelerometer, worn for 7 days. Validated questionnaires measured self-esteem, depressive symptomology, overall emotional and behavioural mental health, internalising problems, externalising problems, and prosocial behaviour. A computerised cognitive test battery assessed executive functions of switching, spatial working memory, and inhibition. Linear mixed models examined associations between activity behaviour compositions and mental health indicators for all participants and separately by primary and secondary school. Post-hoc analyses modelled the influence of reallocating fixed durations of time between activity behaviours on mental health.Results. For all participants ST was associated with worse internalising problems, relative to the other activity behaviours. Among primary school participants, relative to the other activity behaviours, ST was associated with poorer prosocial behaviour, and LPA was associated with worse switching and inhibition test scores. For all participants, reallocating time to ST from sleep and MVPA was associated with higher internalising problems. Among primary school participants, reallocating time to ST from any other behaviour was associated with poorer prosocial behaviour, and reallocating time to LPA from any other behaviour was associated with lower cognitive function.Conclusions. Relative to other activity behaviours, ST and LPA were significantly and unfavourably associated with some mental health indicators. These associations were strongest among primary school participants with no significant relationships observed for the secondary school participants. Replacing MVPA with ST or LPA reflected the greatest unfavourable predicted changes in mental health indicators. Our results provide further evidence for the influence of daily activity behaviours on youth mental health.