Isherwood, who wants to pursue new creative opportunities, is understood to be considering positions that would take him away from the day-to-day running of a company.
According to agency sources, Kevin Roberts, the Saatchi & Saatchi worldwide chief executive, is "in no rush" to find a replacement for the man with whom he has shared a working relationship of more than 20 years. Roberts will consider plans for the role at the beginning of next year.
Isherwood joined Saatchi & Saatchi in Sydney in 1986, was made a creative director in 1988 and was promoted to his current role in 1996.
Since then, he has set up the influential Saatchi & Saatchi New Directors' Showcase and has been instrumental in managing the turnaround of the Publicis Groupe-owned network, alongside Roberts.