Bob Reitemeier, chief executive of the Children’s Society, will leave the charity in November after a decade at the helm.
The charity announced on Wednesday that Reitemeier would join the Essex Community Foundation as its chief executive in January.
Reitemeier joined the Children’s Society as its operations director in 1998 and became chief executive in 2002.
A statement from the charity said he would be credited with moving the organisation into a new area of working for social justice.
Bishop Tim Thornton, chair of the Children’s Society, said Reitemeier had been an inspirational leader who would leave a lasting legacy.
"Under his leadership, the Children’s Society defined its vision of how it would work with and for the most disadvantaged children and young people into the 21st century," he said. "He has put the the voice of the child at the heart of all our work and created major change for the nation’s next generation."
Reitemeier said: "I have been privileged to have been part of this magnificent organisation and feel incredibly proud of what we have achieved on behalf of the thousands of disadvantaged children and young people we serve."
A spokesman for the charity said it was planning to appoint an interim chief executive while it looked for a permanent replacement. He said trustees were yet to decide whether the interim person would be recruited internally.