The effects of low income on children's adjustment might be accounted for by disruptions to hypothalamic–pituitary–adrenal (HPA)‐axis activity and to the development of effortful control. Using longitudinal data and a community sample of preschool‐age children (N = 306, 36–39 months) and their mothers, recruited to over‐represent low‐income families, we explored the associations among diurnal cortisol levels and effortful control, and we tested a model in which diurnal cortisol and effortful control account for the effects of family income on child adjustment. Continuous indicators of morning cortisol level and diurnal slope, as well as dichotomous indicators reflecting low morning levels and flat diurnal slope, were examined as predictors of rank‐order changes in two dimensions of effortful control, executive control and delay ability. Low income was related to a flat diurnal cortisol slope, and above the effects of family income, a flat diurnal cortisol slope predicted lower social competence. Low morning cortisol level predicted smaller gains in executive control and higher total adjustment problems. Further, delay ability predicted lower adjustment problems above the effects of income and diurnal cortisol levels. The results suggest that HPA‐axis dysregulation and effortful control contribute additively to children's adjustment. Copyright © 2013 John Wiley & Sons, Ltd.