Mark O'Neill


Consultant cardiologist and professor of cardiac electrophysiology


Mark O'Neill completed his undergraduate medical and postgraduate scientific training at University College Dublin and the University of Oxford before embarking on specialist medical and cardiology training in London.

In 2005, he undertook a specialist training fellowship in cardiac electrophysiology in Bordeaux with Professors Michel Haissaguerre and Pier Jais, specifically to develop expertise in catheter ablation of atrial fibrillation.

He joined Guy's and St Thomas' in 2009 where he now leads the clinical research group in cardiac electrophysiology.

He is a council member of Heart Rhythm UK, the training lead for EP in the London Deanery and directly supervises students towards the PhD degree at King's College London, in addition to maintaining a full clinical workload as an NHS consultant. He is a regularly invited faculty member for major international arrhythmia meetings and is widely published.

Education and training

  • 2012 Fellowship of the Royal College of Physicians
  • 2011 Fellowship of the Heart Rhythm Society
  • 2007 Certificate of Completion of Specialist Training (Cardiology)
  • 2001 Member of Royal College of Physicians, London
  • 1998 MB BCh BAO (first class honours) University College Dublin
  • 1995 DPhil in Physiological Sciences, Rhodes Scholar, University of Oxford


  • 2009 – present: Consultant Cardiologist and Electrophysiologist, St. Thomas’ Hospital, London
  • 2008-2009: Senior Lecturer and Honorary Consultant Cardiologist, St. Mary’s Hospital, London

Specialist Registrar

  • 2007-2008: Academic Clinical Lecturer in Cardiology, St. Mary’s Hospital, London
  • 2005-2007: BHF International Fellowship, Hôpital Cardiologique du Haut Lévêque, Bordeaux
  • 2003-2005: SpR in Cardiology, St. Mary’s Hospital, London
  • 2002-2003: SpR in Cardiology, Central Middlesex Hospital, London
  • 2001-2002: SpR in Cardiology, Hillingdon Hospital, London

Senior House Officer

  • 2000-2001: Whittington Hospital (ITU, Gastroenterology, Endocrinology, Respiratory)
  • 1999-2000: Brompton Hospital (cardiology), Hammersmith Hospital (renal, rheumatology)

House Officer

  • 1998-1999: Intern to the Medical and Surgical Professorial Units, Mater Hospital, Dublin

Research interests

The Clinical Arrythmia Research Group seeks to understand better the mechanisms which cause cardiac arrhythmias in humans, in particular atrial fibrillation and tachycardia. There are a number of key areas:

  • Comparison of fibrosis determined by MRI and electrical conduction properties.
  • Evaluation and study of ablation lesions using MRI.
  • Use of new catheter technologies for ablation.
  • We believe that an interdisciplinary approach will accelerate progress, ultimately leading to better, safer and more permanent treatments for patients with cardiac arrhythmias.


1998 First class honours MB BCh BAO, 1998 Ruben Harvey RCPI prize for highest aggregate mark in Medicine, Surgery & Obstetrics in Dublin Medical Schools, 1941 Medal in Medicine, Mater Hospital Gold medal in medicine, University Gold medal for first class honours in finals (1998), Surgeon Hugh Kennedy Boyle bequest for best performance in Surgical finals (1998), Colman Saunders medal in Paediatrics, Kirwan medal and prize & Dwyer Joyce Prize in Ophthalmology, Dr Henry Hutchinson Stewart Prizes in Physiology (1992), Biochemistry (1991) and Pharmacology (1992), Dillon memorial prize in Pharmacology (1992), Health Research Board Research Scholarship (1992), Ambrose Birmingham medal in anatomy (1991) First place in year, 5 of 6 years of UCD medical school 1992 Rhodes Scholarship, University of Oxford

Last updated: April 2022

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