Jan Tregelles, the chief executive of Mencap has announced her intention to step down from the role in the autumn.
Tregelles, who became chief executive of the learning disability charity in 2013, has worked at the charity for 22 years and was previously the director of the charity’s personal support division.
A spokeswoman for the charity said Tregelles was not retiring, but it was not known where she planned to move to.
In a statement, Tregelles said: "‘Finding the right time to hand over the baton is always difficult, particularly when it involves an organisation and team as inspiring as Royal Mencap."
But she said the charity was approaching the end of the first five-year phase of its Big Plan strategy, published in 2015.
"It therefore feels the right time to hand over to the person who will lead the development and implementation of its strategy for the coming five years," she said.
"Leading Mencap has been a great privilege and I am proud of what we have achieved in recent years."
Tregelles was paid £164,350 in the year to 31 March 2018, according to the charity’s accounts.
The process to appoint her replacement was expected to start in the next month, according to a spokeswoman for the charity.
Derek Lewis, chair of Mencap, said: "Jan has been inspirational in her leadership and in her dedication to the interests of people with learning disabilities over more than two decades."
He credited her work with ensuring that Mencap was among the leaders in the quality of its services.
"As chief executive she has been the driving force behind Mencap’s ambitious Big Plan to reduce discrimination and ensure people with learning disabilities have the same opportunity as others.
"She will be greatly missed and trustees are determined to find a worthy successor. I am very grateful to her for agreeing to remain in post until later this year to ensure that we have a smooth transition."