Dunleavy, who described her departure as ‘transparent and amicable', said that she was in the process of finalising a new agency role, which she was unable to give details about.
‘I felt the next obvious step in my career was to head up an agency, but obviously that role isn't available at Porter Novelli,' she said.
A mutual agreement was reached that Dunleavy would not work out a six-month notice period to avoid disruption to major clients such as the agency's pan-European account for security firm ADT and UK work for British Airways.
Dunleavy was formerly deputy MD of PN's Banbury office and joined the board 18 months ago.
UK managing director Jean Wyllie said that PN was in the process of a recruitment programme at all levels, but was not currently looking for a direct replacement for Dunleavy.