Using a 2-wave longitudinal design, this study examined the role of school engagement in the association between students' implicit self-theories and internalising and externalising behaviours. A sample of 1 070 Chinese high school students completed surveys on implicit theory of intelligence, school engagement, and internalising (i.e., depression and anxiety) and externalising (i.e., aggressive and delinquent behaviour) problems. The results from structural equation modelling analysis showed that self-theories were positively associated with internalising (i.e., depression and anxiety) and externalising (i.e., aggressive and delinquent behaviour) problems. Moreover, the three dimensions of school engagement (i.e., behavioural, emotional, and cognitive) were mediators between self-theories and internalising problems. Students with implicit self-entity theories exhibit less engagement in behaviour, emotions, and cognition, associated with more internalisation issues. Self-theories were related to externalising problems through the behavioural and cognitive engagement, respectively. Students with implicit self-entity theories exhibit less behaviour and cognition engagement, associated with more externalising behaviours. These results are consistent with Dweck's social-cognitive theory of motivation on the role of the implicit self-theories in adolescents' school engagement and mental health problems.