Professor Sullivan, a practising consultant, is Associate Professor in Paediatric Gastroenterology at Oxford University. He is Associate Dean (Postgraduate Medicine), and Director of Learning and Teaching in Paediatrics at the University of Oxford Medical School. He is also Director of Medical Education (Associate Medical Director) of Oxford University Hospitals NHS Foundation Trust.

His expertise covers the full range of gastrointestinal problems in children, with particular expertise in the gastrointestinal and feeding problems of children with neurodisability especially cerebral palsy. Professor Sullivan is actively engaged in research in this area in which he has produced numerous scientific papers and textbooks.

In November 1991 he was awarded certificates of accreditation in both general paediatrics and paediatric gastroenterology by the Royal College of Physicians Specialist Advisory Committee of the Joint Committee on Higher Medical Training.

Professor Sullivan initiated and developed the paediatric gastroenterology service at the John Radcliffe Hospital, Oxford. Annually, the services provides for around 2000 outpatient referrals, 500 inpatients, and the performance of over 200 gastrointestinal endoscopies.

He has been an Officer of the Royal College of Paediatrics and Child Health including a term as Chairman of the College Special Advisory Committee on Paediatric Gastroenterology, and has held the roles of Convenor of the British Society of Paediatric Gastroenterology Hepatology and Nutrition, and President of the Commonwealth Association of Paediatric Gastroenterology.