Sir Muir has worked for the National Health Service in England since 1972, occupying a variety of senior positions during that time, including serving as the Director of Research and Development for Anglia and Oxford Regional Health Authority, and first establishing and  then being the  Director of the UK National Screening Committee. He founded the National Library for Health, and was the Director of Clinical Knowledge, Process, and Safety for the NHS (England) National Programme for IT, serving as the Director of the National Knowledge Service.  He was the first person to hold the post of Chief Knowledge Officer of the NHS (England), also serving as the co-Director of the Department of Health’s Quality Innovation Productivity and Prevention  (QIPP) Right Care Programme.  Together with Sir Iain Chalmers, Muir was instrumental in establishing the Cochrane Collaboration.