Additive and bidirectional effects of executive control and hypothalamic–pituitary–adrenal (HPA) axis regulation on children's adjustment were examined, along with the effects of low income and cumulative risk on executive control and the HPA axis. The study utilized longitudinal data from a community sample of preschool age children (N = 306, 36–39 months at Time 1) whose families were recruited to overrepresent low-income contexts. We tested the effects of low income and cumulative risk on levels and growth of executive control and HPA axis regulation (diurnal cortisol level), the bidirectional effects of executive control and the HPA axis on each other, and their additive effects on children's adjustment problems, social competence and academic readiness. Low income predicted lower Time 4 executive control, and cumulative risk predicted lower Time 4 diurnal cortisol level. There was little evidence of bidirectional effects of executive control and diurnal cortisol. However, both executive control and diurnal cortisol predicted Time 4 adjustment, suggesting additive effects. There were indirect effects of income on all three adjustment outcomes through executive control, and of cumulative risk on adjustment problems and social competence through diurnal cortisol. The results provide evidence that executive control and diurnal cortisol additively predict children's adjustment and partially account for the effects of income and cumulative risk on adjustment.