Mitchell will be responsible for growing the media group's search offering across its agencies OMD UK, Manning Gottlieb OMD, M2M, PHD, PHD North and Rocket, which have 85 search specialists in total.
He will report to Philippa Brown, the OMG chief executive, and take responsibility for a claimed £60 million in search spend.
In his previous role, which he occupied for two years, Mitchell led the team that brought in the international search contract from Europcar Group, the first SEO-only account win at OMG. Before joining OMD, Mitchell spent six years at the digital marketing agency The Search Works.
Brown said: "I'm looking forward to Mark applying his skills and experience across the group to help drive our already flourishing search business to even greater success."