Paul trained in medicine at University College London. After a stint in hospitals around the UK, he went to Papua New Guinea as the medical officer in charge of Raihu Hospital in West Sepik, and then as a clinical epidemiologist at the PNG Institute of Medical Research

After six years in PNG, he moved to London, where he worked in urban health at the London School of Hygiene and Tropical Medicine with Professor Trudy Harpham in the Health Policy Unit. He then joined the Liverpool School of Tropical Medicine as Head of the International Health Group.

Paul was a founding member of the Cochrane Collaboration. He ran the Cochrane Infectious Diseases Group (CIDG) for 25 years, with a portfolio of over 180 reviews, 20 editors, and 400 authors; and a large programme of research across 10 countries spanning 1994 to 2024. As coordinating editor, he commissioned reviews, trained authors, supported review production, managed quality assurance, and ensured updates were made when needed. He worked closely with the WHO Guidelines on malaria, TB, HIV, and organisation of care, and served on the GRADE Guidance Group (G3) for several years.

The CIDG published some pivotal reviews, including one on impregnated mosquito nets for malaria, that helped underpin the current global distribution of 2 billion nets worldwide since 2004. Paul himself authored reviews on Directly Observed Therapy in tuberculosis and public health programmes for soil-transmitted helminths. He was the corresponding author on the individual patient data meta-analysis that underpinned the WHO recommendation to adopt artemisinin combination treatments.

He was the first person to report symptoms now known as the post-COVID-19 condition. He recovered using beliefs and techniques based on a contemporary understanding of the brain's response to infection. He now works to interpret research reliably on the mechanisms and treatment of people with chronic fatigue conditions.

He has been a Clarivate Highly Cited Researcher since 2023; he is a Trustee with Sense about Science, a member of the Steering Group of the Oslo Chronic Fatigue Network, and co-director of the Recovery Channel Community Interest Company.