Marshall, who reports to the Aegis chief executive, Nigel Sharrocks, will be responsible for ensuring the best possible service and experience for clients working with Aegis companies, which include Carat and Vizeum. He will work three days a week.
He will advise client teams, mentor senior management and work with group companies to lead the development of clients' communications strategies.
Sharrocks said: "Jim is a real industry leader who I've known for many years and I'm delighted that he's joining Aegis Media UK.
"His tremendous wealth of senior experience will be incredibly beneficial in ensuring our clients' needs are always fulfilled and our service levels are second to none."
Marshall left Starcom MediaVest Group in September after 28 years.
He said: "This is a terrific opportunity for me to work for the most progressive media agency group and to work with people I've known, liked and respected over a number of years (very many years in some cases). And not forgetting, of course, a fantastic selection of clients."
Last week, Marshall was appointed as a non-executive director for the CRM start-up Media By Permission.