Objective: To investigate a framework of correlates of both mental illness and wellbeing in a large, current, and nationally representative sample of children in the United Kingdom. Method: An ecologic framework of correlates including individual (sociodemographic and human capital), family, social, and wider environmental factors were examined in 12,347 children aged 11 years old from the UK Millennium Cohort Study. Mental illness and wellbeing scores were standardized to allow comparisons, and the variance explained by the different predictors was estimated. Results: Mental illness and wellbeing were weakly correlated in children (r = 0.2), and their correlates were similar in some instances (e.g., family structure, sibling bullying, peer problems) but differed in others (e.g., family income, perceived socioeconomic status, cognitive ability, health status, neighborhood safety). The predictors included in the study explained 47% of the variance in symptoms of mental illness, with social relationships, home environment, parent health, cognitive ability, socioeconomic status, and health factors predicting large amounts of variance. A comparatively lower 26% of the variance in wellbeing was explained by the study variables, with wider environment, social relationships, perceived socioeconomic status, and home environment predicting the most variance. Conclusion: Correlates of children’s mental illness and wellbeing are largely distinct, stressing the importance of considering these concepts separately and avoiding their conflation. This study highlights the relevance of these findings for understanding social gradients in mental health through the life course and the conceptualization and development of mental illness and wellbeing in childhood as precursors to lifelong development in these domains.