The council says that since Mr Hill became executive director for children, young people and learners, there have been significant improvements in school exam results.
It adds that he has overseen reductions in pupil absentee rates and the introduction of a programme to reform social care provision for children.
Mr Hill, who last financial year earned £141,789.60, has also played a key role in driving forward the council's plans for transforming secondary education in the borough.
Tim Pollard, the council's cabinet member for children, young people and learners, thanked Mr Hill for his contribution.
But he added: "We are very disappointed he is leaving well before we have completed our improvement programme.
"The Essex position is a significantly larger role and we therefore wish Dave all the best for the future."
The council is making an immediate start on the search for a successor and expects to announce interim management arrangements within a few weeks.
Mr Hill came to Croydon in June 2008 from Merton Council.
Croydon was the eighth London borough he had worked for since starting in the public sector as a residential social worker in 1977.
Commenting on his new job, Mr Hill said: "I believe my achievements at Croydon will act as an excellent platform for the challenges awaiting me at Essex."