The majority of mental health conditions emerges before adulthood, with prevalence increasing especially during childhood and adolescence. Coupled with high comorbidity between disorders, there is an urgent need to identify transdiagnostic mechanisms that underpin both risk and resilience to developmental psychopathology. Here we propose executive functions (EFs), a set of cognitive processes relevant for goal-directed control of thought and actions, as a promising candidate for such a mechanism. We review evidence that EF development is shaped by both genetic and environmental factors and relies on protracted maturation of neural networks, particularly frontoparietal and cingulo-opercular systems. Disruptions in these systems are associated with elevated risk for internalising and externalising disorders. We therefore propose EFs as a cognitive endophenotype of developmental psychopathology-heritable, measurable, and lying on the pathway from gene to disorder. We review evidence showing that impaired EF is a shared feature across psychiatric conditions and predictive of future symptomatology, supporting its transdiagnostic relevance. We also propose a mechanistic refinement of the EF construct, moving beyond capacity-based interpretations to a model in which EF performance reflects dynamic and contextually informed cost-benefit computations. Disruptions in reward and effort processing-manifesting as anhedonia and apathy-are common across mental health conditions and may compromise EF performance by altering these computations. This integrative perspective helps explain the limited efficacy of traditional EF training interventions on improving mental health symptoms and underscores the importance of motivational and contextual factors. We conclude by outlining a future research agenda focussed on disentangling the specific subprocesses underpinning EFs and their mechanistic links to developmental psychopathology. Recognising EFs as a transdiagnostic cognitive endophenotype governed by rational control processes opens new avenues for precise, developmentally informed, and targeted interventions in youth mental health.