Lancashire County Council said Chief Executive Angie Ridgwell will step down to take up a new job at Hertfordshire County Council.
It did not name a successor.
Ridgwell, a qualified accountant, worked at Lancashire County Council since 2018, steering the authority through the pandemic and working with cabinet and senior leaders.
“I have loved my time at Lancashire County Council and like to count myself as an honorary Lancastrian,” said Ridgwell. “Whoever replaces me will have the pleasure of working with some of the most committed and passionate people I have ever met.”
The council said it will now begin the recruitment of a new chief executive and ensure a smooth transition.