BACKGROUND: Cognitive ability and socioemotional behaviour during childhood have independently been shown to impact educational outcomes. The extent to which their co-development predicts these outcomes remains unclear. This study aimed to assess associations between concurrent cognitive and socioemotional development trajectories in childhood and exam results at age 16 years. DATA AND METHOD: We analysed longitudinal data on 9084 children from the UK Millennium Cohort Study. Exposure trajectories of cognitive and socioemotional development from age 3 to 14 years were characterised using group-based multi-trajectory models. We used logistic regression to assess associations between these development trajectories and exam attainment, measured by passing five or more GCSE subjects at age 16, adjusting for confounders. Population-attributable fractions were calculated to quantify the contribution of cognitive and socioemotional problems to poor educational outcomes. RESULTS: Compared with the 'no problem' trajectory group, the odds of not achieving a standard pass in GCSE was 2.5 times higher for the 'late socioemotional problems' trajectory group (adjusted OR 2.5, 95% CI 2.1 to 3.1) and four times higher for the 'early cognitive and socioemotional problems' trajectory group (adjusted OR 4.2, 95% CI 3.4 to 5.3). The OR was highest for the trajectory group with persistent cognitive and socioemotional problems (adjusted OR 4.4, 95% CI 3.3 to 5.8). Approximately 17% of poor exam results in adolescence were attributable to cognitive and socioemotional behaviour problems in childhood (adjusted population-attributable fraction 17%, 95% CI 15% to 19%). CONCLUSION: In a representative UK cohort, adverse development of cognitive and socioemotional behaviour in childhood was associated with a negative impact on exam results in adolescence, more so when the adverse development co-occurs or clusters early or persistently. Cross sector health and education policy that invests in reducing cognitive and socioemotional behaviour problems in children has the potential to improve educational outcomes in adolescence.