There were six children in the family: Mary and Elizabeth (twins), Clement, (known as Jack), Lilian, Alfred, and Lucas (known as George).

[1] After graduation, De Garis travelled to the Australian outback to take up her first full-time position in Muttaburra, north-west Queensland.

In 1911 Mary De Garis travelled to the outback town of Tibooburra, western New South Wales to work at the hospital until 1915.

[citation needed] Wishing to be closer to her fiancé, Colin Thomson, who had enlisted for the Australian Imperial Forces, in 1916 she sailed back to London and worked at the Manor Hospital for five months.

Conducting research into the causes of pain in labour and other obstetric matters, she published over 40 articles and letters in the British/Australian Medical Association journals as well as 5 books.