While practicing mindfulness can potentially mitigate and prevent mental health problems among adolescents, mindfulness programmes delivered in schools do not uniformly lead to uptake of mindfulness practice. This low adherence threatens the internal validity of mindfulness trials, and may hinder the alleviation of mental health problems in youth who fail to take up potentially effective techniques. Consequently, it is vital to investigate what predicts uptake of independent mindfulness practice in such interventions. This study investigates whether social cognitions from the Reasoned Action Approach and initial mental health predict mindfulness practice among 1,646 adolescent recipients of the school-based Healthy Learning Mind mindfulness intervention. Path analyses revealed that, in line with the Reasoned Action Approach, descriptive and injunctive norms, and positive and negative outcome expectations predicted intention to practice mindfulness (R-squared = .37, p<.001), which in turn predicted different measures of mindfulness practice itself (R-squared = .09-.17, p<.001). Neither perceived behavioural control nor mental health variables (depressive symptoms, internalisation and externalisation of difficulties, and resilience) were associated with mindfulness practice after the intervention (R-squared = .01, p>.05). Social norms and outcome expectations are potential intervention targets to increase mindfulness practice motivation and behaviour among adolescents.