In his new role, Marshall will have responsibility for the agency's operations in the capital, reporting to the global president, John Hilbrich.
He takes over from Catherine Gale, the European chief executive of The Marketing Store, who left in May.
Rather than directly replacing Gale, the agency has decided to appoint a figurehead for its UK operations to focus on its biggest clients, which include Shell, Vodafone, GlaxoSmithKline and McDonald's.
Marshall resigned from Publicis Dialog in May after seven years, when it was rebranded as Publicis Modem.
Marshall said: "The Marketing Store is in a strong position with its pedigree in promotional marketing and retail. I've been impressed with the vision and expectations of the Chicago management team, as well as the strength of the staff in place in London."
Hilbrich said: "Simon has a formidable track record, he will be an inspirational leader for us in the UK. He has huge experience of building profitable relationships with clients as well as structuring businesses for growth."
Marshall will be joining the company three weeks after The Marketing Store won a four-way pitch to handle Vodafone's UK retail communications and point-of-sale business. The incumbent, RMG Connect, had declined to repitch for the task.