Brennan will be responsible for leading and growing the Omnicom agency’s mobile offering, reporting to Natalie Bell, the head of digital.
He will oversee the mobile and tablet planning and investment teams, delivering mobile media solutions to clients including the John Lewis Partnership, Nissan, and Virgin Group.
Prior to his 16-month stint at Carat, Brennan spent four years at WPP's MEC, rising to become the agency’s emerging platforms director. He completed the IPA Excellence Diploma, the association’s highest qualification, in 2012.
Bell said: "We are delighted that someone as well respected in the industry as Mark is bringing his mobile expertise to Manning Gottlieb OMD.
"It was quite clear from the first time we spoke with Mark that he embodies the culture of our agency. He's passionate about what he does and truly cares about creating the best opportunities for our clients."
Brennan replaces Halliday, who became the Asia-Pacific head of Airwave, Omnicom Media Group’s mobile-specialist arm, in March.