She was born in Sheringham, Norfolk, England, and educated at Oxford High School in North Oxford and the University of Oxford, reading Natural Sciences at Lady Margaret Hall. She then studied medicine at University College, London.
When the Second World War started she was appointed to a post at the Samaritan Hospital. From 1947 she ran a mobile obstetric team from University College Hospital. Barnes was the first woman consultant obstetrician and gynaecologist at Charing Cross Hospital (1954) and the first woman President of the British Medical Association (1979–80). She was also Chairman of the Elizabeth Garrett Anderson Hospital Appeal Trust, President of the Association of Chartered Physiotherapists in Obstetrics and Gynaecology (known since 1994 as the Association of Chartered Physiotherapists in Women's Health) from 1977 to 1995, and President of the Royal British Nurses' Association. She took a prominent role in the public debate over the 1967 Abortion Act. In 1994 she delivered the Hunterian Oration at the Hunterian Society
She had married Brian Warren, a lieutenant in the Army, in 1942.
She was also a Friend of the English Pocket Opera Company and a Guardian of Westminster Abbey. She was a Fellow of the Royal College of Physicians, the Royal College of Surgeons of England, and the Royal College of Obstetricians and Gynaecologists (of which was she sometime Vice-President).
- Listen to an oral history interview with Dame Josephine Barnes - a life story interview recorded for National Life Stories at the British Library
* Munk's Roll of the Royal College of Physicians of England