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Professor Mark O'Neill is a Consultant Cardiologist and Professor of Cardiac Electrophysiology at St Thomas' Hospital and King's College London. He sees private patients at The Cardiac Clinic at London Bridge Hospital.

Professor O'Neill completed a DPhil in Cardiac Physiology at the University of Oxford as a Rhodes Scohlar in 1995 before graduating with first class honours from University College Dublin in 1998. He completed his subspecialty training in interventional cardiac electrophysiology at St Mary's Hospital and in the internationally renowned arrhythmia centre in Bordeaux.

Professor O'Neill was appointed as a Consultant at St Thomas' Hospital and King's College London in 2009. He became a Reader in 2011 and was appointed as Professor in 2013. Since then, he has led the arrhythmia service, the regional training programme and the cardiac electrophysiology divisional research programme. He was awarded a National Bronze Clinical Excellence award in 2016 and 2021 in recognition of the quality and breadth of both his clinical and academic work.

In his current role, Professor O'Neill treats patients with all manner of palpitations, including those caused by simple and complex heart rhythm conditions such as supraventricular tachycardia, atrial fibrillation, atrial flutter/tachycardia, arrhythmias associated with adult congenital heart disease and ventricular tachycardia. Where appropriate, he performs interventional electrophysiology procedures to manage these conditions. He spends time helping his patients to understand fully their symptoms and works with them to develop tailored treatment plans to suit their needs and concerns.

Outside of his clinical work, Professor O'Neill is an international opinion leader in cardiac electrophysiology. He is actively involved in research and has published widely in peer-reviewed journals. His research addresses the mechanisms of atrial fibrillation and the use of imaging and computational modelling techniques to improve clinical outcomes of AF ablation. He lectures regularly both in the UK and overseas on the management of atrial arrhythmias.