Walker - who joined the then Dewe Rogerson a year before its merger with Citigate - has handed in his notice, but is yet to announce his date of departure.
While it is understood he will remain in City PR, Walker is remaining tight-lipped over his next career move.
Citigate managing director of UK PR Patrick Donovan said Walker was 'leaving with our best wishes to pursue other interests'.
Walker - who was unavailable for comment as PRWeek went to press - quit the then Shandwick Consultants, where he was an associate director, to join Citigate in 1997.
He started as a director in that agency's UK City PR team.
Walker's clients include food giant Dairy Crest, which hired Citigate last August. He also assumed responsibility for a short period to head the group's marcoms team, after the departure of comms manager Jemma Smith last June.