J.M.S. Careless was born in Toronto, Ontario and attended the University of Toronto Schools. He received a Bachelor of Arts degree in 1940 from the University of Toronto. He attended Harvard University and received a Master's degree in 1941 and a Ph.D. in 1950.
During the Second World War, he worked in the historical branch of Naval Service Headquarters at Ottawa, then transferred to the Department of External Affairs, where he served as Canadian Diplomatic Officer aboard the exchange ship Gripsholm. Careless began lecturing at the University of Toronto in 1945, where he taught graduate and undergraduate courses in Canadian political, ethnic, urban and intellectual history. He retired in 1984. He was one of the most respected historians of his generation. He was fond of reminding his students that nothing in history is inevitable until it happens. J. M. S. Careless married Elizabeth Isobel Robinson on December 31, 1940. The couple has 5 children and 10 grandchildren.