Martincic will report to Duncan Painter, the chief executive of Ascential, and has been tasked with managing all aspects of brand strategy to help grow the company.
Ascential owns Cannes Lions, the annual festival for the advertising and creative industries, as well as the trade titles Drapers, Nursing Times and Retail Week, and the retail data company WGSN.
Martincic, who will take up the London-based role on 3 October, was most recently FCB’s executive vice president for global branding and corporate affairs.
During a decade at the Interpublic network, Martincic was responsible for developing communications and marketing tools to position the FCB brand across its 120 offices worldwide.
The French native had worked with Ascential last year on its rebrand. The company, previously known as Top Right Group, changed its name in December.
Prior to FCB, Martincic managed corporate communications and new business for Leo Burnett, Starcom and OMD.
Painter said: "I’ve known Stephen for many years and he has consistently earned his reputation as a global brand expert with a passion for creativity and the ever-changing media landscape. After working with him when he led our rebrand last year we realised that his insight and expertise would be of considerable value to our Executive team to drive and support our growth ambitions.
"The skills, expertise he brings and excellent work Stephen has already delivered for us will be key to developing our external brand. It is also vital to help further build upon our internal culture to increase brand and marketing expertise across all our talented teams. I could think of no better choice to welcome to our team."