Michael Rosen, who became DCS at Norfolk County Council in September 2015, said today that he is to move on from the role.
It follows a monitoring visit by Ofsted to keep tabs on progress since the authority was rated "inadequate" last October - the second time it received the bottom rating in the space of two years.
Inspectors said that while there has been "positive movement" in some areas of children's services, the overall pace of change is "slow".
"Leaders have not been ruthless enough in pursuing improvement at all levels," a letter outlining Ofsted's findings to Rosen states.
"Difficulties recruiting high calibre managers and staff, and poorly developed strategic planning and performance management, have hindered the authority's efforts to transform services.
"As a result, children and young people are still not consistently receiving good enough services."
Rosen said children's services in Norfolk have "come a long way" since he started.
"I'm pleased that inspectors have seen improvement in all the areas they reviewed, but there is still more to do and members have indicated a desire for a change in our approach," he said.
"In my view, the immediate improvement priorities for children's services need a director with a different skillset if the continuing challenges in that part of our service are to be addressed at the pace Norfolk's children deserve.
"The urgency with which these challenges need resolving has led to my decision to create space for a new director to be brought in as soon as possible.
"I am sure that the talented and committed staff of the department will respond positively in maintaining their current focus on rapid improvement at all levels, with continued support from members and corporate leadership for the direction of travel."
As part of plans to improve services, Barnardo's is in line to help run looked-after children's services in Norfolk, in what is thought to be the first arrangement of its kind in England.