Douglas joined Cafcass - the Child and Family Court Advisory and Support Service - in September 2004 from Suffolk County Council, where he was director of social care and health.
"My job at Cafcass has been a privilege and our task now, after achieving ‘outstanding' in our recent Ofsted inspection, is to take our service to children to the next level," said Douglas.
"I will continue to give my all until my last day and look forward to helping our next chief executive officer get up and running when the time comes."
At the time he joined Cafcass, the agency was widely seen as failing organisation with many children involved in family court proceedings were waiting weeks or months to be allocated children's guardians.
Under Douglas's leadership, Cafcass improved significantly and in a national inspection of the service published in March this year Ofsted rated the organisation as "outstanding".
"The contribution of the chief executive is highly valued and is considered to be ‘exceptional'," said Ofsted in its report.
"He is held in ‘high regard' by senior judiciary and the Ministry of Justice. Together with his senior team, he works tirelessly to drive much-needed development and reform to meet the ever-increasing levels of demand."
Douglas, who was adopted at birth, started his career as a social worker in 1976 at the London Borough of Hackney. A spokesperson for Cafcass said the recruitment process for Douglas's replacement is yet to begin.