Mild learning disability is associated with an increased risk of affective disorder. This study examines the extent to which adult socio-economic disadvantage and ill health contribute to this risk. Samples were drawn from the 1958 National Child Development Study. Relative to a comparison group, mild learning disability at age 11 was associated with elevated rates of depressive symptoms throughout adult life, and carried a six-fold risk of chronic depressed mood. The group difference in depressed mood at age 43 years was in large part mediated by variations in adult socio-economic disadvantage and ill health.