He joined Batey last June after a recommendation from Jon Collins, chairman of Batey Singapore, who is a former chairman of the Jupiter Group.
"He's still in Singapore considering what he wants to do ... he's looking in the region and outside,
added Marrett.
Mitchell moved to Batey six months ago from George Patterson Bates Sydney, where he was a creative director.
"He has made a significant impact on the brand communications work for StarHub,
said Marrett.
"Chris is as good as anyone I've worked with at Batey in the last eight years."
He said promoting people internally was better for "stability reasons ... (and) Chris has demonstrated creative leadership in the agency and I know he can deliver".
In addition to work on StarHub, Mitchell's new role will involve overseeing creative work for Batey's other clients - Singapore Airlines, Singapore Tourism Board, DaimlerChrysler and banking group OCBC.
Since Mitchell is taking on extra responsibility, Batey is looking for a creative group head to work on the StarHub business. Callaghan was unavailable for comment.