Hewitt will take over financial management of the agency as well as its associated "village" companies in the UK.
His arrival frees up Jerry Wales, the current finance director, who will become the chief financial officer for the group as well as joining the plc board which will come into being after the flotation, probably this autumn.
Hewitt's appointment follows a brief spell as the finance director of the ill-fated Ben Mark Orlando, the short-lived start-up established by his McCann colleague Ben Langdon and Mark Wnek.
As well as spells with McCann and Collett Dickenson Pearce, Hewitt was also the chief financial officer of Hatpin, which owns Kendall Tarrant and White Door.
Nick Hurrell, the M&C Saatchi joint chief executive, said: "Jamie has a lot of experience and is uniquely wise about the industry. He not only knows about advertising, but communication in many of its diversified forms."
M&C announced in March that it was planning to float. Its aim is to provide funds to establish start-up operations in France, Germany, Spain and Italy while allowing the founding partners to realise some of the agency's value, which is estimated at £75 million.