After schooling at home and then the Advanced School for Girls, Mayo commenced studying for an arts degree at Adelaide University in 1896. She wanted to study medicine but her parents believed her to be too young to undertake medical studies. Two years later Mayo began her degree in medicine and in 1902 graduated as the university's second female medical graduate.

Mayo worked at the Adelaide Hospital (now Royal Adelaide Hospital) for a year and then worked overseas for a time at the Hospital for Sick Children, London, Coombe Hospital in Dublin and St Stephen's Hospital for women and children, Delhi. She also completed a course in tropical medicine. Mayo returned to Adelaide in 1906 and established her own practice. She was most interested in midwifery and women and children's health. Mayo and social worker Harriet Stirling established a clinic for mothers and babies in 1909 and this grew to become the School for Mothers' Institute and Baby Health Centre. This, in turn, developed into the Mothers and Babies' Health Association in 1927 which eventually served the whole of South Australia and was the precursor to the current state government department Child and Youth Health.

In 1911 Mayo was appointed clinical bacteriologist at the Adelaide Hospital. In this position she created a vaccine department in the hospital. Mayo founded an independent hospital for babies in 1913 and this was taken over by the government in 1917 as Mareeba Babies' Hospital. During the First World War she was a demonstrator in pathology at the University of Adelaide and from 1919 she was appointed as a physician at the Adelaide Children's Hospital. In 1926 Mayo gained her doctorate and began lecturing in children's diseases at the University of Adelaide. During the Second World War Mayo was senior paediatric advisor at the Adelaide Children's Hospital and coordinator of the Red Cross transfusion service.

Mayo was a foundation member of the Australian College of Physicians (now Royal Australian College of Physicians) and a member of the South Australian branch of the British Medical Association and Australian Paediatric Association.

Mayo was actively involved with the University of Adelaide for most of her life. In 1914 she was elected to the University of Adelaide Council; the first woman to hold such a position in Australia. She served on the council until 1960. Mayo was also instrumental in the establishment of St Ann's University College for Women and was chairperson of its council 1939-1959 (the college is now for both women and men). In 1945 she was president of the Australian Federation of University Women. In Mayo became the first chairperson of the Adelaide Lyceum Club for professional women.

Helen Mayo House, a service for women experiencing mental health concerns after the birth of a child, based at the Women's and Children's Hospital in Adelaide is named in her honour.

1902: Awarded Everard scholarship

1902: Graduated from Adelaide University; the university's second female medical graduate

1906: Established private practice in Adelaide

1909: With social worker Harriet Stirling established clinic for mothers and babies

1911-1933: Clinical bacteriologist at the Adelaide Hospital

1913: Founded independent hospital for babies which later became Mareeba Babies' Hospital

1914-1960: Member of the University of Adelaide Council; the first woman sit on a university council in Australia

1921-1947: Served as honourary consulting physician and honorary responsible medical officer of Mareeba Babies' Hospital

1922: Became first chairperson of the Adelaide Lyceum Club for professional women

1926: Gained Doctorate in medicine (MD) from the University of Adelaide; the first MD conferred on a woman by the university

1926-1934: Clinical lecturer in children's diseases at the University of Adelaide

1927: Established the Mothers and Babies' Health Association; served as honorary chief medical officer until 1967

3 June 1935: Made Officer of the Order of the British Empire (OBE) for services to maternal and child welfare activities

1939-1959: Chairperson of St Ann's University College for Women's council

1949-1953: President of the Mothers' and Babies' Health Association

Hutton Neve, M. (Marjorie). This mad folly : the history of Australia's pioneer women doctors, Sydney : Library of Australian History, 1980, pp. 153-152

Mackinnon, Alison. The new women: Adelaide's early women graduates, Netley, S. Aust. : Wakefield Press, 1986, pp. 60-72

Mayo family. Papers of the Mayo family, PRG 127

Milne, Ann. 'Some educational trends in the History of South Australia', New horizons in education, no. 51 (Autumn 1974), pp. 2-7

'Noted for service to children', The advertiser, 16 November 1967, p. 30

Australian Dictionary of biography Online: Search for Mayo, Helen Mary

Australian Women's Register Search for Helen Mayo

University of Adelaide: See: Latest news and events: Lumen Magazine: Winter 2007 issue: Dr Helen Mayo OBE