Starting with the publishing of his PhD thesis, as The Making of Geology in 1977, Porter wrote or edited over 100 books, an academic output that was, and is, considered remarkable. He is so prolific that the poet Michael Hofmann has called him "a one-man book factory." He is particularly notable for his work in the history of Medicine, in pioneering an approach that focuses on patients rather than doctors. Despite his recognition in the history of Medicine, he is quoted as saying, "I'm not really a medical Historian. I'm a social Historian and an 18th century man". In addition to the history of Medicine and other sciences, he specialised in the social history of 18th-century Britain and the Enlightenment. He also wrote and lectured on the history of London. With G. E. Berrios, Porter published A History of Clinical Psychiatry (1985) and co-edited the international journal History of Psychiatry (1989). He also edited the journal History of Science for many years.