She will work on growing the agency globally with new business and expanding its remit with current clients.
Dixon has also been tasked with "strengthening relationships" with other agencies part of Publicis.Sapient and the Publicis Groupe network.
She will report to Laurent Ezekiel, the global client services director who took on the additional role of managing director for New York and Atlanta last week. Dixon will also have a dotted line to Ewen Sturgeon, the international chief executive at DigitasLBi.
She joined the agency in 2011 as associate client partner, handling the Coca-Cola account. Before this, she worked at Ogilvy Worldwide.
Ezekiel said: "Lianne is a world-class operator who has a brilliant sensitivity for client needs. I’m looking forward to working with her as we continue to service our global clients and set the bar high for client services."