John was an undergraduate in Cambridge before moving to the Oxford Clinical School. After qualification he worked in clinical posts in London before returning to Oxford where he undertook a DM in renal physiology, further clinical training, and post-doctoral research in molecular biology. He was appointed consultant physician in Oxford in 1993, moving to Cambridge in 1996 where he has since undertaken a variety of roles including being Clinical Director of renal services and associate dean of the Clinical School. He is now Deputy Medical Director. He often teaches clinical students and junior doctors in the hospital. He is Director of Clinical Studies in the College, and his teaching was recognized by the University by the award of a Pilkington Prize in 2010.
At a national level, John is Chair of the UK General Internal Medicine Specialist Advisory Committee, which is responsible for organisation and oversight of training in Internal Medicine. He is an expert advisor to a committee of the UK Commission on Human Medicines, which is responsible for assuring safety and efficacy of medicines.