His career has been equally divided between Harvard and Columbia. At Harvard he was Chief of the Pediatric Hematology-Oncology Unit at the Massachusetts General Hospital 1967 to 1988, and Associate Professor at Harvard Medical School. At Columbia he was Chairman of the Department of Pediatrics at its New Jersey campus in Morristown 1988 to 1997, Deputy or Acting Chairman at the Morgan Stanley Children’s Hospital of New York 1997 to 2008, and Professor of Clinical Pediatrics at the College of Physicians and Surgeons. In 2008 he retired to become Professor Emeritus and Special Lecturer in Pediatrics at Columbia, and in 2014 returned to Boston as Honorary Pediatrician at the Massachusetts General Hospital. In 2018 the annual John T. Truman Lectureship in Pediatric Hematology-Oncology at the Massachusetts General Hospital was inaugurated.
He is currently docent at the Paul Russell Museum of Medical History and Innovation at the Massachusetts General Hospital and a member of the Department of Pediatrics.
He is also an enthusiastic harpsichordist, pianist, opera buff, birder and bagpiper.