Alterations to cognitive function are often reported with depression and anxiety symptoms, yet few studies have examined the same associations with mental well-being. This study examined the association between mental well-being, depression and anxiety symptoms and cognitive function in 1502 healthy adult monozygotic and dizygotic twins, and the shared/unique contribution of genetic and environmental variance. Using linear mixed models, mental well-being was positively associated with sustained attention, inhibition, cognitive flexibility, motor coordination and working memory, whereas depression and anxiety symptoms were (...) associated with poorer sustained attention, inhibition, cognitive flexibility and executive function. Bivariate twin modelling showed well-being shared a small environmental correlation with motor coordination and a small genetic correlation with working memory. Trivariate twin modelling showed well-being shared a small genetic correlation with inhibition, whereas depression and anxiety symptoms shared a small environmental correlation with inhibition. The remaining variance was mostly driven by unique G and/or E variance. Overall, well-being and depression and anxiety symptoms show both independent and shared relationships with cognitive functions but this is largely attributable to unique G or E variance and small shared G/E variance between pairs of variables. (shrink)