Having gained his PHD and worked as an academic scientist, Walker started lecturing physics and chemistry at universities in the UK, France and the USA.
During the 1970s he worked for the Greater London Council in the director general's department on housing and associated issues, before joining the Department of Trade in 1977. Between 1987 and 1989 he ran the Alvey programme of collaborative IT research and development. before moving to the department of energy to focus on civil nuclear matters. He was then appointed UK governor of the International Atomic Energy Authority and chairman of the European Bank for Reconstruction and Development Nuclear Safety Account. In 1995 he began working at the Home Office as director general for immigration and nationality. In 1998 he was appointed deputy chairman of Customs and Excise, now HM Revenue & Customs, and was seconded to the office of the government E Envoy where he worked on the Modernising Government Agenda with Local Government Association chief executive Brian Briscoe. In 2000 he was appointed director general of the Health and Safety Executive.
- Trustee, Drinkaware
- Member of the research strategy committee, Prostate Cancer UK
- Fellow, Institution of Engineering and Technology
- Fellow, Institute of Physics
- Brasenose College, University of Oxford