In this earlier portion of his life, de Ropp was active in plant physiology and cancer research. In 1939 he was at the Research Institute of Plant Physiology at Imperial College of Science and Technology in London. In the first quarter of 1939, at Paddington, he married Eileen M. Trinder, with whom he had lived for a number of years. He and Eileen had two children. During the Second World War, de Ropp worked as a bacteriologist and plant biologist. He met Kathleen Elizabeth (Betty) Knowlman when during the War years he carried out research, and she worked as a gardener, at Kew Gardens (England's botanical research and education institution). Betty later joined him in the United States after he moved there, at which time they married.