He will report to managing director Mike Welsh, with a remit to develop new business.
Rees ran B&Q and Vodafone's direct accounts at RMG, prior to which he worked at AIS, Wunderman and BBH, running accounts for Audi, Royal Mail and O2.
Rees said: "Claydon Heeley is one of the few agencies in town that is genuinely geared up to deliver outstanding creative work across both traditional and emerging media. The whole agency understands where direct marketing is heading and that's a very alluring prospect."
The move is designed to bolster Claydon Heeley's digital offering, after a series of account wins including Glenfiddich and Daimler Chrysler. The agency recently poached eight digital creatives from twentysix London, including creative director Maxine Gregson.
Welsh said: "Xav's passion and fired up attitude is exactly what I need in a deputy. His car choice of two Audis does beg a few questions but I'm hoping his appointment as deputy MD will see my golf handicap improve quite quickly."