David Simpson has worked in tobacco control for over twenty years. He is the founder and first director of the International Agency on Tobacco and Health (IATH), which since 1991 has provided information and advice on tobacco control issues to colleagues in countries with fewer resources. Before that, he was director of the United Kingdom's main tobacco control advocacy group, Action on Smoking and Health (ASH), an early example of an organisation established by doctors to fight against tobacco.

He has long been active internationally with the World Health Organisation (WHO) and the International Union Against Cancer (UICC), on teaching, consultancy and training work in over 30 countries. He is a member of the management committee of GLOBALink, and News Analysis editor of Tobacco Control, the BMJ specialist journal. He is also an honorary member of the United Kingdom Faculty of Public Health Medicine, and an honorary consultant at the Clinical Trials Service Unit (CTSU) and Imperial Cancer Research Fund's (ICRF) Cancer Studies Unit in Oxford.