General Counsel Cindy Laquidara is retiring from City Hall, effective July 1.
Laquidara will be joining the Jacksonville office of Akerman Senterfitt, a law firm with 20 offices across the country.
She has been with the city since 1998, working with the mayoral administrations of John Delaney, John Peyton and Alvin Brown.
She said the work has been "highly rewarding" but the time is right personally for her to make a career move.
The announcement was at a Wednesday afternoon news conference that included Brown, Clerk of Courts Ronnie Fussell and JEA CEO Paul McElroy.
Brown said Laquidara has had many successes in her career, most recently the $13.4 million settlement the city received over the embattled Shipyards property.
The mayor will appoint an acting general counsel, but he declined to say who.