Helen Marion Macpherson Mackay (23 May 1891 – 15 July 1965) was a British paediatrician. She made important contributions to the understanding of childhood nutrition, and preventive healthcare. Mackay was the first woman fellow of the Royal College of Physicians. After graduation from the London School of Medicine for Women (now part of the UCL Medical School), Mackay practiced at the Queen Elizabeth Hospital for Children where she was the first female physician to work there, and also one of the first women to be appointed as a consultant there. Helen Mackay was the first person to investigate anaemia in infants and to attribute iron deficiency as an important factor of this. This led to changes in childhood nutrition favouring breastfeeding over formula feeding in the United Kingdom (UK).