John was an undergraduate in Cambridge before moving to the Oxford Clinical School. After qualifying 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 working in Prof Sir Peter Ratcliffe's laboratory in the early years of his exploration of the cellular oxygen sensing mechanism.
He was appointed consultant physician in Oxford in 1993, moving to Cambridge in 1996 where he has undertaken a variety of roles including being Clinical Director of renal services and associate dean of the Clinical School. He is now Cambridge University Hospital's Deputy Medical Director and Responsible Officer (lead for professional governance and liaison with the General Medical Council). He often teaches clinical students and junior doctors and is Director of Clinical Studies in the College. His teaching was recognized by the University by the award of a Pilkington Prize in 2010.
At a national level, John was for many years Chair of the UK General Internal Medicine Specialist Advisory Committee, which is responsible for organisation and oversight of training in Internal Medicine.