- Simon has previously served as:
- Head of the Cabinet Office COVID-19 Taskforce, from March 2020 to March 2022
- Director General, Withdrawal Agreement, Transition Taskforce, from January 2020 to March 2020
- Director General Future Economic Partnership, Department for Exiting the European Union, from July 2019 to January 2020
- Director General, Decentralisation and Growth, Ministry of Housing Communities and Local Government, from 2015 to 2019
- Chief Executive, Planning Inspectorate, from 2014 to 2015
Simon served as Acting Permanent Secretary at the Home Office from 28 March until 14 April 2025.