Professor Warner completed his undergraduate medical training in the School of Medicine, University of Sheffield and his initial paediatric experience was at the Children''s Hospital, Sheffield working under the mentor-ship of Professor Ronald Illingworth.
He acquired his MRCP(UK) having worked for two years in adult medicine with a particular focus on respiratory disease. He then moved to the Hospital for Sick Children, Great Ormond Street as a Registrar and subsequently a Research Fellow and then Senior Registrar rotating through Queen Elizabeth hospital, Hackney and the London hospital.
His first consultant post was a joint appointment between St Mary's and the Brompton hospital. In 1982 he helped establish the London Allergy Clinic before moving to Southampton as Professor of Child Health in 1990 and finally back to London as Professor of Paediatrics and Head of Department at Imperial College St Mary's hospital campus from September 2006 to April 2013. He is also Hon Professor of Paediatrics in the University of Cape Town.
In 2008 he became Director of Research for the Women and Children's Clinical Programme Group, Imperial College Healthcare NHS Trust (ICHT). He was the lead for paediatrics in both the Biomedical Research Centre in ICHT and the NW London CLAHRC (Collaboration for Applied Health Research and Care) and was President of the Academic Paediatrics Association.
Professor Warner's research has focused on the early life origins of asthma and related allergic and respiratory disorders. He has published over 400 papers in scientific journals on these topics. He was Editor in Chief of the journal Paediatric Allergy and Immunology from 1997-2010 and chairman of the paediatric section of the British Society for Allergy and Clinical Immunology for 5 years until 2010. He was also a member of the Speciality and Training committee of the World Allergy Organisation and a past Trustee of the charity known as The Anaphylaxis Campaign.
He was a member of the Advisory Committee for Novel Foods and Processes of the Food Standards Agency for 12 years until 2012 and was recognised for his work in food allergy research by the award of an OBE in 2013.