Hannan was one of the first women to study in the Royal College of Surgeons in Ireland.

In 1894 she became Chief medical officer in the Lady Dufferin Hospital in Shikarpar, Upper Sind.

On the census for 1901 she has a young patient staying with her and a governess and nurse from India as well as the domestic servants.

Hannan lectured in midwifery and served as medical officer at the Victoria Maternity Hospital from 1914 to 1921.

She lived for a period of the twenties, 1924–1927, in Johannesburg but the rest of the time was in Pretoria.