Barbara Peacock, Medway council's director of children and adults services, is to leave for a new £165,000 role.
News was announced yesterday, by Croydon Council, that Mrs Peacock would be leaving Medway to become Croydon's new executive director for people.
Medway council leader Alan Jarrett said it had come as a shock when Mrs Peacock handed in her notice last week, but he wished her luck in the new job.
"I think with senior directors they're always looking to better themselves which is why they're in the position they are," he said. "Congratulations to her. She's done a good job for us."
Mrs Peacock started her £121,000-a-year role with Medway in 2012, leading one of the council's biggest single briefs - including schools, adoption, fostering, adult social care, youth work and physical and learning disabilities.
Medway has since been issued with improvement notices in 2013 and 2014 as a result of poor performance in children's social care services; while the Towns' key stage 2 are among the worst in the country.
But Cllr Jarrett said her work overall had improved her department.
"We've just come out of intervention," he said. "Ofsted have given us a good bill of health.
"There's never a good time for a senior director to leave, but we're in a better place than we were. She's improved the area she's responsible for.
"The problem is finding a replacement at that level."
Mrs Peacock will leave in three months time, and her role will be taken up temporarily by her current deputy Ian Sutherland.
"He's a top bloke," added Cllr Jarrett. "We'll see how that goes and we're lucky to have him."