Gillie was the eldest among four children of Dr. Robert Calder Gillie, a minister in the Presbyterian Church of England, and Emily (née Japp) Gillie. She studied at Wycombe Abbey School, University College London and University College Hospital, graduating in 1925. In 1927 she became member of the Royal College of Physicians. She started working as assistant to a medical partnership of three women, and after they retired continued receiving patients at her home in Connaught Square in London. During World War II she moved with her two children to a country cottage at Pangbourne, continuing her medical work until retirement in 1963.