Rob Williams, chief executive of Bliss, is leaving the premature baby charity to become chief executive of the Office of the Children's Commissioner.
The charity is advertising for a successor on a salary of about £62,000.
Williams, who worked for Concern Worldwide before joining Bliss three years ago, has never worked in the statutory sector.
He will leave a staff of 32 at Bliss at the end of July to head a 26-strong team in his new position, where he will work alongside Al Aynsley-Green, the Children's Commissioner for England.
"The Government can learn a large amount from the voluntary sector about how it deals with clients," said Williams. "My job will be to make sure the office knows exactly what children and young people are thinking and that the department is led by their views."
He added that it was unusual for voluntary sector leaders to switch to senior public sector positions, but he said he hoped his experience in the charity sector would help him in his new role at the office, which is supposed to be independent of government.
"The Office of the Children's Commissioner is a broader agenda than the premature baby agenda, and that's what attracted me to it," he said.
Bliss has doubled in staff and increased its turnover from £1.2m to £2.2m since Williams arrived.