She began working as a general practitioner in Aberdeen, focusing on the health of the city's working class, and developed an interest in social work.
In 1915, she was the first Chief Medical Officer of the Scottish Women's Hospitals for Foreign Service in Troyes, alongside Louise McIlroy. She was next appointed to the Royal Army Medical Corps, with the title of Controller of Medical Services (Overseas).[2] However, she refused to take up the post on the grounds that her pay would be 60% less than she could earn as a general practitioner, and was less than a man with the same rank. She finally took up the position in 1917, and was given responsibility for operations in France.[4] She was known for avoiding wearing a military uniform, instead working in a skirt, shirt and tie.[5]
Sandeman also served on the Scottish Board of Health Consultative Council.[6]
Death
On 22 February 1929 she died of pneumonia in Aberdeen.[7]Mary H. J. Henderson, administrator of the Scottish Women's Hospitals in Serbia, and a war poet, paid a tribute to Sandeman, along with Dr. Elsie Inglis, founder of Scottish Women's Hospitals, in Magdalene in her 1929 book, Warp and Woof, when saying:
^"Books that Should Please as Christmas Gifts - Notable Contributions to Northern Verse - Warp and Woof by Mary H.J.Henderson". Aberdeen Press and Journal. 19 December 1929. p. 9.