[1][2][3] Dunn read for a BSc in biology at the University of Birmingham and then an MSc in ecology at Durham University, where her thesis was on "the affects [sic] of habitat fragmentation on the woodland edge micro-climate and on the structure and composition of woodland ground flora",[4] after which she joined the Department for Environment in 1995 as a policy adviser.
After three years, she was appointed as the next Head of the Economic and Domestic Affairs Secretariat, replacing Jonathan Slater in 2016.
[2] In Summer 2018, it was announced that Dunn would be promoted to replace Patsy Wilkinson as the Second Permanent Secretary for the Home Office.
[1] On 29 February 2020, she was appointed acting permanent secretary at the Home Office, following the sudden resignation of Sir Philip Rutnam.
[7] In April 2021 Dunn left the Home Office to become Second Permanent Secretary at the Department of Health and Social Care.