The 2013 Lancet series on maternal and child undernutrition highlighted maternal depression as a significant risk factor for poor child growth and recommended interventions to address the problem through maternal and child health and nutrition programs. As such, this study assessed associations of maternal common mental disorders (CMD) with undernutrition, diarrhoea, and acute respiratory infections in children aged 0–5 years in Bangladesh, Viet Nam and Ethiopia. Maternal CMD, which affected nearly half of women in Bangladesh and one-third in Viet Nam, was shown to be an important determinant of child stunting and underweight, respectively. No such association was found in Ethiopia, although CMD affected 39% of women. Maternal CMD was strongly associated with childhood illnesses in all three countries. Interventions to support maternal mental health are important for women's own well-being and could make important contributions to improving child health and nutrition. These findings suggest that a potential pathway for the relationship between maternal CMD and child undernutrition could be through child illness – as high levels of maternal CMD may hinder a mother’s ability to take adequate care of her child, prevent illnesses and seek health care when the child is ill, hence leading to higher illness rates.