Having ceded creative command of the UK's fifth-largest shop to Nik Studzinski last summer, Moira said that he had decided to make a "clean break" because he missed direct involvement in the creative output.
Moira's decision ends an association with Publicis that stretches back 20 years, 12 of which were spent in charge of the creative department.
Moira, 53, will leave almost immediately, precipitating a search by the agency for an external candidate to replace him as chairman.
A hugely popular industry figure and one of its wittiest commentators, Moira added the chairmanship to his creative responsibilities in July 2002, succeeding Richard Hytner, now the head of the Saatchi & Saatchi European network.
He took over the chairman's role full time in the middle of last year after Dave Droga's arrival as Publicis' worldwide creative director. Droga decided to install Studzinski, the former Saatchis head of art, as the executive creative director of Publicis' London shop.
However, Moira admitted that he had been unfulfilled by his role as an agency "ambassador" and new-business catalyst and had spent a month mulling over his future.
"The chairmanship is a terrific job -- but for somebody else," he said. "I've given it a go but I never really fancied trying to be a suit. I'd like to sleep around, creatively, for a while in the hope of finding the people I love and settling down."
Grant Duncan, the Publicis chief executive, acknowledged it would have been hard to meet Moira's desires with a new creative director at the helm of the agency. "Gerry casts a long shadow and it would have been difficult and intimidating for Nik having Gerry as just another creative when he used to be the boss," Duncan said. "He has been a very active chairman but he misses the gut feel of the creative work."
Moira, whose agency career spans more than 30 years, first joined the then McCormick Publicis from Ogilvy & Mather as the creative director in the early 80s. He left in 1987 to help found Woollams Moira Gaskin O'Malley with his managing director, Chris Woollams. He resumed creative control of Publicis in 1995 after the departure of John Kelley and is credited with establishing the agency's reputation for hard-working and populist advertising for clients such as Renault and Asda.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.