Professor John Marshall is the Frost Professor of Ophthalmology and Chairman of the Academic Department of Ophthalmology, at St Thomas' Hospital, and was formerly Sembal Professor of experimental Ophthalmology at the Institute of Ophthalmology from 1982-1991. His research over the past forty years has ranged over a number of ocular problems but has concentrated on the inter-relationships between light and ageing, the mechanisms underlying age-related, diabetic and inherited retinal disease, and the development of lasers for use in ophthalmic diagnosis and surgery. This work has resulted in almost four hundred research papers and numerous book chapters and books. It produced and patented the revolutionary Excimer laser for the correction of refractive disorders with in excess of 30 million procedures now having been undertaken worldwide. It also created the world's first Diode laser for treating eye problems of diabetes, glaucoma and ageing. He is editor and co-editor of numerous international journals.

He has been awarded the Nettleship Medal of the Ophthalmological Society of the United Kingdom, the Mackenzie Medal, the Raynor Medal, the Ridley Medal, the Ashton Medal, the Ida Mann Medal and the Lord Crook Gold Medal of the Worshipful Company of Spectacle Makers, the Doyne Medal of the Oxford Congress, the Barraquer Medal of the International Society of Cataract and Refractive Surgery, the Kelman Innovator Award of the American Society for Refractive and Cataract surgery, and the Lim Medal of the Singapore National Eye Centre.

He is currently a director of Ellex Medical Lasers and he sits and chairs many national and international committees.