Brian MacMahon (23 August 1923 – 5 December 2007) was a British-born American epidemiologist who chaired the Department of Epidemiology of the Harvard School of Public Health from 1958 until 1988. Best known for his work on the epidemiology of breast cancer, he also pioneered research on associations between passive smoking and lung cancer, and between diet and risk of cancer.

MacMahon was born in Sheffield,^[1] where his father, Desmond MacMahon, was a professional violinist.^[2] In 1948, he married Heidi Marie Graber from Switzerland (died 2001); the couple had two sons and two daughters.^[2] MacMahon also had a brother, Paddy, who was a doctor in the British Army.

MacMahon emigrated to the United States in the late 1950s, and became a U.S. citizen in 1962.^[3] He died at Boston, Massachusetts in 2007, aged 84, following a stroke.^[4]

MacMahon studied medicine at the University of Birmingham, gaining the diplomas of the Royal College of Physicians and Royal College of Surgeons in 1946, and the MB BChir in 1948.^[1] After working as a locum doctor in impoverished areas of Birmingham, MacMahon served as a ship's doctor in the British Merchant Navy from 1946 to 1948.^[1][4] He later said that these experiences left him "somewhat dispirited about a future in clinical medicine".^[1]

Enrolment in a course in public health at the University of Birmingham brought him into contact with epidemiologists Thomas McKeown, Ronald Lowe and Reginald Record, who became his supervisors in a PhD in "social medicine" (as epidemiology was then known) studying infantile pyloric stenosis.^[1][4] After gaining his PhD in 1952,^[3] he travelled to the US to obtain a Master's in epidemiology at the Harvard School of Public Health (1953). In 1955, he gained the MD degree at the University of Birmingham.^[1]

MacMahon's early academic positions were at the University of Birmingham, UK, and at the Department of Environmental Medicine and Community Health at the State University of New York in Brooklyn, USA, where he worked with Duncan W. Clark, focusing on the epidemiology of leukemia and breast cancer.^[1]

In 1958, he was appointed head of the Harvard School of Public Health, USA, a position he held until his retirement in 1988.^[1] In 1967, he co-founded the Society for Epidemiologic Research, a learned society for epidemiologists.^[5] In 1976, he was appointed the Henry Pickering Walcott Professor of Epidemiology at the school, and he also served as the Associate Dean (1977–78). From 1974, he held a professorship in public health at the University of Hawaii at Manoa.^[3] MacMahon built the Harvard School of Public Health into a world-class research institution; he particularly prided himself, however, on nurturing students, many of whom went on to become leaders in the field of epidemiology.^[1][2] Well-known former students include Dimitrios Trichopoulos and Walter Willett.^[4]

MacMahon co-authored the textbook Epidemiologic Methods (1960), later reissued as Epidemiology: Principles and Methods, with Thomas Pugh. Described by Willett as "the first modern epidemiology textbook", it became a standard text in the subject.^[1][2] He co-edited Preventive Medicine (1967), later republished as Preventive and Community Medicine, with Duncan W. Clark.^[6]

MacMahon was awarded the National Divisional Distinguished Service Award of the American Cancer Society (1971), the John Snow Award of the American Public Health Association (1980), the Donald Reid Medal of the London School of Hygiene and Tropical Medicine (1987) ^{[citation needed]}, and the Charles S. Mott Prize of the General Motors Cancer Research Foundation for his work on the epidemiology of cancer (1992). He became a member of the Institute of Medicine in 1973, and later received honorary doctorates from the University of Birmingham, University of Athens and the State University of New York.^[1][3]