Dr. Clark was born in Scotland. His family moved to Canada in 1841, where he was educated at the Simco Grammar School and studied philosophy and mathematics in Toronto. He studied medicine at the Toronto School of Medicine and received his M.D. degrees from Victoria University (1858) and the University of Toronto. He then spent a year in Edinburgh, London, and Paris to further his medical education.
In 1859, he opened a practice in Princeton, Ontario. When the U.S. Civil War started, he joined the Union Army under General Grant. He returned to Toronto after the war. In 1875, he was appointed Medical Superintendent of the Toronto Asylum for the Insane (although he had no previous psychiatric experience), remaining until 1905. He became an expert witness in medical legal cases, wrote on psychiatric subjects, and lectured in medical school classes. He was opposed to the theory of brain localization, and it was said that “he belonged to a school of thought pretty largely his own.” Dr. Clark served on the Ontario Medical Council (1872–1880).
Dr. Clark made frequent contributions both to medical and general literature, including a novel dealing with the Canadian Rebellion of 1832. He was a professor of mental diseases at the University of Toronto. Dr. Clark was President of the American Medico-Psychological Association (1891–92).