Joyce Moseley, chief executive of Catch22, is to leave the young people's crime reduction charity at the end of this year.
Moseley has been chief executive since its formation in 2008, and before that was chief executive of Rainer for nine years, before its merger with Crime Concern to form Catch22.
In an email to colleagues, Moseley said Catch22 was in a very strong state to face the period of upheaval the sector was experiencing.
"We are prepared for the opportunities even as we deal with disappointments, and my successor will be taking on an energised, professional, entrepreneurial organisation that believes nothing is impossible when it comes to making a difference to the lives of the young people, families and communities we are here to serve," she wrote.
Last year Catch22 and the social care charity Turning Point were part of a consortium led by the services company Serco that won a £415m contract to build and operate a new prison at Belmarsh in south London. The role of the charities was said to focus on prisoner resettlement.
Jim McKenna, chair of Catch22, said Moseley had led Catch22 brilliantly, steering it from a £5.5m organisation as Rainer in 1999 to the £50m organisation that it is today.
"This substantial and careful growth has been due to introducing businesslike approaches to Catch22, including better marketing and contracting, a focus on performance management, increased and more diverse fundraising, organisational efficiencies underpinned by a strong set of values that recognise the importance of staff and volunteers," he said.
Moseley added that she would continue with other interests, including her role as non-executive director of the Tavistock and Portman NHS Foundation Trust, as chair of the directors of HCT Group, a transport social enterprise, and as a trustee of the Social Research Unit at Dartington.