He formerly headed MindShare's Ford UK account, where he was most recently responsible for the car giant's multimillion-pound sponsorship of Premier League football on Sky.
Kayali, who has been with MindShare since its inception in 1998, will work with Simon Pardon, the director of trading at GroupM EMEA, and Alan Hodge, the deputy director of trading. He will be based in the MindShare Worldwide office in London.
Nick Waters, the chief executive of MindShare EMEA, said: "Haldun is a top-class trader with considerable experience leading the negotiations and delivery for some of our most important clients in the UK market; he is now stepping up to handle our international clients. He has demonstrated great dedication to his clients and this agency, and considerable vision as to how his area of expertise will develop in the marketplace."
Kayali said: "The Exchange offers advertisers one focal point - with experts in trading all forms of value exchange between brands, consumers and the media - not only the traditional channels, but also the likes of branded-content distribution, editorial marketing and word-of-mouth marketing.
"I'm thrilled to take this role and look forward to expanding those opportunities."