As the chief marketing officer, she is Luca Lindner's first hiring since his arrival a month ago as the president and chief executive of D'Arcy's Europe, the Middle East and Africa operations.
Robertson's responsibilities will embrace not only new business but internal communications and PR. She will also be part of an "inner cabinet which will also include Lindner as well as a chief strategic officer and a chief operating officer who have yet to be named.
A former head of new business for Bates Europe, she will take over European new-business development at D'Arcy from Andy Byrd, who switches to international account management.
Her appointment comes as D'Arcy attempts to re-energise its senior management in the wake of its firing by Mars.
Lindner said: "D'Arcy is a network that's 90 per cent focused on clients and 10 per cent on new business and growth. We have to re-focus without taking our eyes off the clients. Kate has not only worked on international account management and European new business but has tremendous energy and enthusiasm. We need more people like her."
Robertson, a South African, set up the London office of the Cordiant-owned Scholz in 1997. Since then the agency has taken over Herman Beasley and CKMP and has added names such as Siemens, Cable & Wireless and The Wall Street Journal Europe to its client list.
Roberston said: "I felt I'd done what I set out to do at Scholz and my strength has always been in international business. D'Arcy is a mighty challenge but I like Luca. We're both mavericks."