Dr Leong is a surgeon in private practice. He is currently Chairman of the Hong Kong AIDS Foundation.

Dr Leong was an unofficial Member of the Executive Council of the Government of the Hong Kong Special Administrative Region from 2005 to 2012. He was also an elected member of the Legislative Council from 1988 to 2000. Dr Leong has held many key posts in the public service sector, including Chairman of the Hospital Authority (2002-2004), Chairman of the Council on Human Reproductive Technology (2001-2010), Chairman of the Elderly Commission (2005-2011), Chairman of the University of Hong Kong (HKU) Council (2009-2015), Chairman of ICAC Complaints Committee (2010-2015), Chairman of Elder Academy Development Foundation (2009-2017) and Chairman of the Committee on Standard Working Hours (2013-2017).

On the professional side, Dr Leong is the Hunterian Professor of the Royal College of Surgeons of England and a Regent of the Royal College of Surgeons of Edinburgh. Dr Leong was the Founding President of The College of Surgeons of Hong Kong. He served as President of the Hong Kong Academy of Medicine from 2000 to 2004.