He will oversee the commercial operations and financial management of the group, which includes the main London agency as well as Dialog, Modem and Blueprint.
As well as helping the agencies to co-ordinate and work more effectively together, he will also support Richard Pinder, the chief operating officer of Publicis Worldwide and chief executive of Publicis UK, in the day-to-day management of the group.
Pinder said: "Now that we have leaders for each unit in the UK, I want to be sure that, day to day, these managers have someone to help them work closer together. Graham has proven to be a great commercial partner and I'm delighted to recognise that."
Stewart has worked within the Publicis Groupe for almost 20 years. He joined Publicis UK as the chief financial officer in 2005, moving from Starcom EMEA.