Rosalind Franklin worked in Paris before joining King’s College London. She moved there in 1947 to take a postdoctoral position at the Laboratoire Central des Services Chimiques de l’État.
In Paris, Franklin worked under Jacques Mering, a specialist in X-ray crystallography. The research strengthened her expertise in using X-ray diffraction to study the structure of materials. She investigated substances including carbon and graphite, building the technical skills that later became central to her DNA research.
Franklin enjoyed the scientific atmosphere in France and became highly accomplished as a crystallographer during her Paris years. Her time there also helped her develop professional confidence and independence after the frustrations she had experienced in earlier research work in Britain.
She returned to England in 1951, joining King’s College London as a research associate. There she applied X-ray diffraction to DNA and produced crucial evidence about its helical structure, including the famous Photo 51. Paris is therefore the canonical answer; Rome, Vienna, and Berlin were not the city where she held this important pre–King’s College research position.