Clarke will be the president and executive creative director of the agency, formerly known as Modem Media, and will be supported by Brian Dargan, who arrives from DraftFCB as planning director. In addition, Miles Welch, is joining from Endemol as general manager.
The trio will sit on a new London leadership board, along with Chris Ryan, who has been promoted to client services director, and Ian Nutt, who has been made group project director.
The appointments are the first to be made by the Digitas Global chief executive Alan Rutherford, who joined the Publicis Groupe-owned digital network in June, following nine years as vice-president of global media at Unilever.
Rutherford said: "Chris' appointment signals our commitment to putting creative thinking at the heart of our business. I'm convinced that, alongside the new board signings, he will drive the success of Digitas."
Clarke added: "I'm thrilled to take the reins in London as the Digitas network rolls out. We will be focusing single-mindedly on our creative product, with operational and strategic support from Miles and Brian."
The moves fill the hole created when Johnston quit as president of Digitas London to join MindShare Interaction as the joint chief executive for EMEA (Campaign, 6 July).