After graduating, Mayo spent two years working in infant health in England, Ireland and British India.

This organisation, which became the Mothers' and Babies' Health Association in 1927, eventually established branches across South Australia and incorporated a training school for maternal nurses.

[5] In 1906, Mayo returned to Adelaide and started a private practice in premises owned by her father on Morphett Street, next to the family home.

In it, she addressed the high infant mortality rate in South Australia,[1] and claimed that more needed to be done to educate women for motherhood.

The Kindergarten Union made a room in its offices available for one afternoon a week, where a nurse would weigh babies and Mayo and Stirling would give advice.

Mayo served as the honorary medical officer of the association until her death in 1967,[2] by which time the organisation gained a training school for maternal nurses and a hospital.

[5] After visiting Melbourne to learn how to make vaccines,[6] in 1911 Mayo was appointed clinical bacteriologist at the Adelaide Hospital, a position she would hold for 22 years.

The board rejected the proposal, so Mayo and her group rented a two-storey house in St. Peter's and opened a hospital for infants in 1914.

However, following a recommendation by Adelaide surgeon Henry Simpson Newland, Mayo applied for the post, and that year was appointed honorary Assistant Physician in charge of outpatients.

[11] She used her experiences as a clinical bacteriologist at the Adelaide Hospital as the basis for her thesis, which she was forced to write on the weekends, such was the volume of her workload.

[2] She was subsequently appointed honorary physician to inpatients at the Children's Hospital,[2] and a clinical lecturer at the University of Adelaide.

[11] In May 1935 Mayo was appointed an Officer of the Order of the British Empire (OBE) "for services in connection with maternal and child welfare in the state of South Australia.

[2] Dr Elma Linton Sandford-Morgan (22 February 1890 – 1983[13]), author of ABC of Mothercraft,[14] was appointed medical officer for MBHA in 1937.

[18] The construction of the Lady Simon Building for the Women's Union was due in large part to her efforts,[19] as was the founding of St. Ann's College, where she served as chairperson from 1939 to 1959.