Department of Health and Social Care second permanent secretary Shona Dunn is to leave the civil service to become chief executive of St John Ambulance.
The first aid charity said Dunn – who served as second perm sec at the Home Office from 2018 to 2021 – would start in her new role in September.
St John Ambulance chair Stuart Shilson said the organisation felt "extremely fortunate" to have appointed Dunn.
"She has genuine commitment to the things we care about at St John, and the skills and temperament to lead us into the future," he said.
Dunn said she was "thrilled" to be joining the charity at what she called a "crucial time" in its long history.
"Through nearly 30 years of public service, I have seen the importance of supporting communities to stay safe and healthy, and the impact it can have when that comes under strain," she said.
"The role that St John people play, day in day out, in supporting communities up and down the country has never been more important.
"I am delighted to have the opportunity to work with every one of our dedicated volunteers and employees to ensure that this cherished and valuable institution is able to serve and support communities for generations to come."
Dunn joined the civil service in 1995, starting work in the then-Department for the Environment. She subsequently worked at the Department for Education and as director general of the Economic and Domestic Secretariat at the Cabinet Office.
Dunn served as acting perm sec of the Home Office in 2020 after Sir Philip Rutnam quit, accusing allies of then-home secretary Priti Patel of mounting a "vicious and orchestrated briefing campaign" against him.
Dunn moved to DHSC in spring 2021 after her predecessor as second perm sec, David Williams, was promoted to take the helm at the Ministry of Defence.