Modem Media and Publicis Dialog's digital offering is being repackaged as Publicis Modem, which has aligned itself with the Publicis Groupe's advertising network, Publicis Worldwide.
Digitas will offer digital assistance to the rest of the Publicis Groupe. Publicis Dialog will continue to provide other marketing services.
Marshall joined the direct agency seven years ago, soon after it was founded. A Publicis spokesman said Dialog will react to Marshall's departure by strengthening its management group, but not with a CEO level replacement.
Marshall, who is not believed to have another job lined up, said: "Publicis is moving towards a more unified management structure and after seven years at the helm of Publicis Dialog it seemed a good time to consider what I want to do in the future."
Dennis Kerslake, Publicis Dialog's chairman, said: "Simon has played a huge part in the success of Publicis Dialog over the last seven years and he leaves with our thanks and best wishes for whatever challenges he chooses to take on in the future."
Martin Reidy, the former president of Modem, will take over as the chief executive of Dialog Worldwide and Publicis Modem, with a long-term plan to make Modem responsible for 25% of the group's revenue.