Robert Davies Defries was this country's first professionally trained public health administrator. He is probably best known as the co-creator, associate director, and later director of the University of Toronto's Connaught Laboratories and School of Hygiene. The Connaught Laboratories and the School of Hygiene were founded on a threefold premise of providing affordable preventive medicines to the public, researching new medicines, and training future public health administrators. Consequently, this organization came to have a profound effect on the formation of Canadian health care policy. One of the many public healthissues that it directly influenced was mass immunization. Through its co-operation with the City of Toronto's Department of Public Health, Ontario's provincial health department, and other local health departments,the School of Hygiene and Connaught Laboratories effectively orchestrated the first successful mass immunization campaign in Canada against the disease diphtheria. The strategy used in that campaign paved the way for future mass immunization programs and a shift in health policy focus towards disease prevention. Defries was someone who wore many hats. The roles of administrator, educator,  activist, scientific researcher, social planner, editor and writer all contributed to his long and illustrious career. This essay focuses on the early life and career of R.D. Defries (1889-1936 approximately), and explores how his passionate religious idealism and public health convictions combined to shape his views of society and himself