Mary is a British physician, trained at Girton College, Cambridge and Birmingham University Medical School. She worked for 7 years in the British National Health Service before being awarded a Wellcome Trust scholarship in 1984 to study the microflora of the small intestine in diarrhoea at the Instituto de Investigación Nutricional (IIN) in Peru.

Mary returned briefly to the UK but was committed to continuing research in Peru and returned in 1989 and has remained since. She was involved with the Young Lives project from the beginning and was a Co-Principal Investigator for Young Lives from Round 1-5 in the IIN, and was responsible for the health and nutrition aspects of the project in Peru. She was director of the IIN and runs other projects mainly in relation to child health and nutrition, improving health services and conducting clinical trials.