Johnston will now look at acquisition and expansion opportunities across Europe for MindShare, in a bid to expand the range of digital services that the network offers.
He will report to Nick Waters, the chief executive of MindShare EMEA, and joins Rob Noss, the MindShare Interaction chief executive, who will now share the role and take responsibility for managing global clients.
Waters said the agency was looking to expand from a digital media agency into a provider of more diverse digital services, such as eCRM and communications planning.
"Clients are increasingly looking for a broader range of services. Johnston's appointment is a step towards increasing our capabilities in that regard," he said.
Johnston, who will also be staffing-up MindShare Interaction, said: "A lot of the innovation that is happening in digital over the next few years will happen in media."
Johnston leaves Digitas London, formerly known as Modem Media, after ten years - latterly as the managing director of Modem Media in Europe. Modem was rebranded in May after it was acquired by Digitas and following Publicis' subsequent £1.3 billion purchase of Digitas last year.
Johnston, who is expected to start his new role in the autumn, plans to work with Alan Rutherford, the chief executive of Digitas Global, to ensure a smooth transition until his replacement is found.