In the early 1940s, de Ropp wrote a number of research papers relating to plant physiology and tumours. By 1945 he was a Research Officer of the Agricultural Research Council at the Rothamsted Experimental Station. After emigrating to the U.S., de Ropp's professional life included a stay at the Rockefeller Institute as a visiting investigator. At various times, his research was centered on cancer, mental illness, or drugs that affect behavior. During a ten-year period working for the Lederle Laboratories near Pearl River, New York, de Ropp wrote a book for the general reader in the field of psychoactive substances (many of which are plant-derived): "Drugs and the Mind".