Mark Lever is to leave the National Autistic Society in May after 11 years as chief executive.
Lever, who led the campaign for the Autism Act 2009, has been appointed chief executive of Helpforce, a community interest company that develops volunteering in the NHS.
He said in a statement that public awareness and understanding of autism had been "transformed" over the past decade.
"I feel it is the right time to pass the baton to a new leader who will continue to grow the services we provide and continue to improve society’s attitudes towards autism," he said.
Before joining the society, Lever rose from director of training to chief executive at the Royal Voluntary Service, which was then known as the WRVS.
Carol Homden, chair of the trustees at the National Autistic Society, said: "Mark has been an exceptional leader and we are fortunate to have had him at the helm of our charity for such a long period.
"We know that much remains to be done to make society work for autistic people and we are looking forward to recruiting a new chief executive who can help us to take the next steps towards achieving this."
The society said in a statement: "The charity will now be looking for a new chief executive to build on the organisation’s new brand and take advantage of the increased public and political interest in autism."
Homden will lead the recruitment process for Lever's successor.
It is expected that there will be a gap of a few months between when Lever leaves and the new person takes up the role, during which some other members of the charity's senior management group and trustees will be expected to provide cover.
Founded in 1962, the society campaigns for autistic people’s rights and provides support and advice to autistic people and their families.
According to documents filed with the Charity Commission, its income was £96.1m in the year ending 31 March 2018 – down for the second consecutive year from £98.7m in 2016.
It employs more than 3,100 people.