John Kellett graduated from Trinity College Dublin in 1970 and completed his internship at Mountainside Hospital in Montclair, New Jersey, USA. He subsequently worked for one year as a general practitioner in a small Newfoundland outport before completing a residency in internal medicine at Dalhousie University, Nova Scotia, Canada.
In 1976, he moved to Winnipeg, where he completed a four-year fellowship in endocrinology. During this period, he conducted research on growth hormone and growth factors. He returned to Ireland in 1980 as a consultant physician at Nenagh Hospital, County Tipperary.
He is a Fellow of the Royal College of Physicians of Ireland, the Royal College of Physicians and Surgeons of Canada, the European Federation of Internal Medicine, and the American College of Physicians.
His main research interests include medical decision-making, acute medical care, prognostication, and predictive models. He has authored over 200 peer-reviewed publications and remains actively involved in several collaborative research projects, including medical device development, early warning scores, and intelligent systems.
He has held academic appointments at the University of Manitoba, Dundalk Institute of Technology, Ireland, the Northern Ontario School of Medicine, and the University of Southern Denmark. He is currently a Visiting Professor at the University of Greater Manchester.