The agency says the move is part of an effort to put creativity at the heart of its thinking as it develops an all-round communications offering flexible enough for the current tough trading conditions.
The change has been precipitated by the imminent departure of Richard Hytner, the current chairman.
He is to take 12 months' sabbatical to join the London Business School's Sloan Masters MSc programme with a view to taking a broader role within the Publicis Groupe, soon to be enlarged with the addition of Bcom3.
Moira's additional responsibilities coincide with the appointments of the managing director Grant Duncan and Derek Morris, as joint chief executives.
Morris, the former Unity partner, joined Publicis in April in the newly created role of chief strategic officer. The new chief executive pairing will combine Duncan's account management experience with Morris' expertise in communications strategy.
The new management line-up is completed by John Wisbey, the chief operating officer and the head of Publicis Dialog, into which all the Publicis below-the-line interests are grouped.
His task will be to encourage clients to make more use of the group's diversified services and to develop the group's new-business strategy.
In appointing Moira as chairman, Publicis follows a trend set by TBWA/London, M&C Saatchi and Grey, which have also moved their respective creative chiefs, Trevor Beattie, James Lowther and Tim Mellors, into similar roles.
Duncan said: "Gerry becoming chairman reflects our desire to put our foot on the creative accelerator and bring creativity back to the centre of our activities."
Moira will not relinquish command of the creative department, although more of his workload will be taken on by his creative directors, Adam Kean, Alex Taylor and Keith Courteney.
Moira said: "It's no fluke that we're now a top-five agency, and these changes show we've every intention of staying there."