In this newly created role, Brennan will report to Francesca Bateman, head of mobile development at Carat, and will be consulting with clients to build content on the mobile platform, drive investment and deliver better business value.
Brennan joins Carat from MEC where he was emerging platforms director, responsible for growing and testing new technology, and boosting clients' mobile investment.
He introduced mobile, gaming, connected TV and social media to an impressive client list from Paramount Pictures, Nintendo and Wilkinson Sword, and was shortlisted for the highly acclaimed MediaWeek Rising Star Award last year.
Tracey de Groose, managing director for Carat, said: "It's great to have Mark on board to develop the role of mobile within our clients' communications plans.
"Across market sectors, advertisers are increasingly recognising the potential of using mobile in an integrated way and Carat is committed to offering a market-leading service in this area, working closely with Aegis specialist agency Isobar Mobile."
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