Dr. Leong is a medical doctor in private practice specialising in urology. Dr. Leong is one of the most respected medical professionals and public health administrators in Asia, having actively engaged in various influential public roles during his career. He was a member of the Legislative Council of Hong Kong, representing the medical functional constituency from 1988 to 2000. In 2002, Dr. Leong was appointed as the chairman of the Hong Kong Hospital Authority where he was involved in drawing up the blueprint for Hospital Authority, taking special responsibility for Medical Service Development. Subsequently, Dr. Leong served as a non-official member of the Executive Council HKSAR from 2005 to 2012. He was chairman of the Hong Kong University Council from 2009 until 2015 and spearheaded the commissioning of The University of Hong Kong – Shenzhen Hospital in Shenzhen during his term. Dr. Leong also served as the Chairman of the Standard Working Hours Committee. His other former public positions include the Chairman of the Elderly Commission HKSAR, Chairperson of the Council on Human Reproductive Technology and Chairman of the Elder Academy Development Foundation.
Dr. Leong worked in the department of surgery in HKU for 16 years, he is an Honorary Professor of Surgery at HKU and an Honorary University Fellow of HKU. In recognition of his contributions to the medical profession and to the community of Hong Kong, HKU has conferred upon him the degree of Doctor of Science honoris causa in 2006. In addition to training medical students, Dr. Leong also conducted cutting-edge research in his specialist field of urology. His major research contributions include experiments and the human use of the stomach for bladder replacement and urinary diversion. Dr. Leong was appointed Hunterian Professor by the Royal College of Surgeons of England in 1975. He was the first Hong Kong-born Chinese doctor to have received this accolade. Dr. Leong played an instrumental role in the establishment of the Hong Kong Academy of Medicine and became its President from 2000 to 2004. Dr. Leong is the founder of the Hong Kong Kidney Foundation and the Hong Kong Kidney Action Group which promotes vital organ donation. He also founded the International Drug Abuse Treatment Foundation to study the efficacy of Traditional Chinese Medicine in treating addiction. As Chairman of the Hong Kong AIDS Foundation, he spearheaded a Beijing Hong Kong AIDS Cooperation and the China AIDS network to help promote the necessary AIDS education for workers in China. He is Chairman of the Society for the Promotion of Hospice Care and has consolidated the Asia Pacific Hospice Palliative Care Network, establishing a foothold for hospice service in the Asia Pacific region. Dr. Leong was conferred an Honorary Doctor of Social Science by the City University of Hong Kong in 2019. He is an independent director of New Frontier Health Corporation (stock code: NFH.NYSE), a company listed on the New York Stock Exchange.