After training posts at Cardiff and Cambridge, he undertook research into large intestinal function at The Dunn Nutrition Unit, Cambridge. For this he was awarded his MD degree and the Sir Lionel Whitby Medal of the University of Cambridge. He then took up a post as Lecturer in Medicine at University College Hospital. A year was spent gaining extra experience in liver disease at the Yale University Medical School Liver Disease Unit in the USA. Ian McNeil was appointed as Consultant Physician to Ealing Hospital in 1989.
Following his appointment at Ealing, he led the fund raising and development of the hospital's endoscopy unit. As well as practicing in gastroenterology he has had responsibilities for the acute general medical service in the hospital. He has been Clinical Director of Medicine, Chair of the Drug and Therapeutics Committee and Chair of the Medical Staff Committee at Ealing. Before recently leaving Ealing Hospital, he was Lead Consultant for Endoscopy.
Dr McNeil is a Fellow of the Royal College of Physicians for whom he has been a member then Secretary of the Gastroenterology Committee, and both question setter and examiner for the MRCP examination. He has served on committees of the British Society of Gastroenterology.