Geisler will be based in London and will report to Alan Rutherford, the former global media chief at Unilever, who was appointed as the chief executive of Digitas Global a year ago.
Geisler will be responsible for the Publicis-owned digital network's global clients, such as Delta, General Motors and Procter & Gamble.
Geisler has been at Saatchis, which is also a Publicis-owned network, for slightly more than a year. He was previously the global business director at Wunderman, the WPP-owned direct marketing network.
Digitas was acquired by Publicis Groupe in December 2006 for $1.3 billion as part of the holding company's plans to expand its digital expertise.
Since Rutherford joined, in April 2007, Digitas has rapidly grown its presence outside of the US, where its business operation had previously been focused.
In May 2007, the London office of Modem Media became part of Digitas, and the network has since extended its reach into China, Japan, France and Singapore.