After taking a BSc degree in Anatomy (1967; 1^st Class Honours) at London University, he qualified in 1970 with distinctions in medicine and pathology at the London Hospital Medical College. After internships in medicine and surgery at the London Hospital, and a year in pathology and morbid anatomy at the Bernard Baron Institute of Pathology, The London Hospital, he trained in general medicine and neurology at The Hammersmith Hospital, and University College Hospital and then spent 2 years as Lecturer in Medicine and Neurology Specialist at the University of Dar es Salaam, Tanzania.

He completed his neurology training at Atkinson Morley’s Hospital and St George’s Hospital London and then undertook a PhD in neuroscience with Professor David Marsden FRS at the Institute of Psychiatry, London.

After consultant and Senior Lecturer posts at the Wessex Neurological Centre in Southampton and St George’s Hospital London, in 1989 he was appointed to the University Chair of Neurology and Head of the Department of Neurology at The Institute of Psychiatry and King’s College School of Medicine and Dentistry.

There he built a large academic department with an international reputation in neuroimaging, molecular genetics, molecular pathology, and clinical trials focusing on the causes, mechanisms, and treatment of neurodegenerative disorders, particularly Amyotrophic Lateral Sclerosis (Motor Neurone Disease; ALS/MND) and on Parkinsonian disorders, especially Progressive Supranuclear Palsy (PSP) and Multiple System Atrophy (MSA). He was closely involved with the pivotal trial demonstrating that riluzole prolongs survival in MND. Licensed in1996, riluzole remains the only proven disease-modifying drug treatment for ALS/MND.

Seminal work in the department on molecular genetics led by Professors Chris Shaw and Ammar Al-Chalabi identified key genes and mechanisms causing and modifying the features and progression of ALS/MND. Professor Steve Williams established a world-leading Centre for Neuroimaging research and Professors Brian Anderton, Simon Lovestone, and Leigh established an MRC Centre for Neurodegeneration Research.

In 1990 Leigh established The King’s MND Care and Research Centre which in 1995 was recognized as the first MND Association-funded MND Centre in the UK. The King’s model of multi-professional coordinated care for ALS/MND has been widely adopted and there are now over 20 such MND Centres around the UK.

He has received The Forbes Norris Award of the International Alliance of MND/ALS Associations, the Erb-Duchenne Prize of the German Neuromuscular Society, and The Sheila Essey Prize of the American Academy of Neurology. He was elected Fellow of the UK Academy of Medical Sciences in 2003. He remains Emeritus Professor at King’s College London.

Having retired from KCL in 2010, he was then appointed Professor of Neurology at Brighton and Sussex Medical School (BSMS) and Honorary Consultant Neurologist at Sussex University Hospitals Foundation Trust where he established and led the Sussex MND Care and Research Network from 2016-2021.

From 2015-2021 he was Chief Investigator and Co-Coordinator of an EU H2020-funded trial (MIROCALS: Modifying Immune Responses and Outcomes in ALS) of low dose interleukin-2 in ALS and continues as co-coordinator of the MIROCALS Consortium. The results of the MIROCALS trial were published in the Lancet in May 2025 (https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(25)00262-4/fulltext).

He co-founded the European ALS Consortium (now ENCALS), served as Scientific Chair of the International ALS/MND Symposia, and has served on many national and international research committees including the MRC and Wellcome Trust Neurosciences Panels. He recently Co-Chaired the Multiple Sclerosis Society’s Research Strategy Committee, and Chaired The PSP Association Research Committee. He currently Chairs The MND Association’s Health Care Research Panel and serves on the Scientific Advisory Board of the MyName’5Doddie Foundation.

He has authored or co-authored over 450 scientific papers and book chapters.