Harris leaves Carat next week.
Emery said: “We’re delighted that James is joining us. The central question that all our clients face is uniting best-in-class performance skills with brand strategy.”
The appointment comes as Mindshare seeks to retain its global American Express account in a review, having lost HSBC to PHD in May.
In a long career in media, Harris has worked with brands including Dell, Coca-Cola, Diageo, General Motors and Mastercard. As well as co-founding MediaCom North in 1996 and Diffiniti (now iProspect) in 2005, he has held senior digital roles at UM and IPG Mediabrands and was previously head of agencies, international, at AOL.
Harris also wrote Think More Analogue, Be More Digital, which was published in 2015.