Dean was educated at Oxford University and St. Thomas's Hospital, London, qualifying with a conjoint diploma in 1904. He was elected to the chair of pathology and bacteriology at the University of Sheffield in 1912, and in 1915 became professor of pathology at the University of Manchester. His early research interests were in immunology. During the First World War, he worked at the 2nd Western General Hospital in Manchester, and was active in dealing with the influenza epidemic. Dean expanded the teaching of pathology and bacteriology at Manchester during the early 1920s before moving to the University of Cambridge in 1922, where he spent the rest of his career.