Moderate inverse correlations are typically found between well-being and mental illness. We aimed to investigate the role of genes and environments in explaining the relationships between two aspects of well-being and two measures of internalizing symptoms. Altogether, 4700 pairs of 16-year-old twins contributed data on subjective happiness and life satisfaction, as well as symptoms of depression and emotional problems. Well-being was moderately correlated with internalizing symptoms (range = 0.45, 0.58). Multivariate twin model-fitting indicated both genetic and environmental overlap. Life satisfaction and happiness demonstrated different patterns of overlap, with stronger genetic links between life satisfaction and depression. Non-shared environmental influences were largely specific to each trait. This study supports the theory of mental health and illness being partly (but not entirely) correlated dimensions. There are also significant genetic and environmental factors to identify for well-being that go beyond the absence of mental illness. It is therefore possible that different interventions are needed for treating mental illness and promoting mental health.