Cooper is currently the Hong Kong-based chief executive of Omnicom Media Group Asia. Pattison left PHD in April to become the chief executive of ILG Digital, the parent company of digital agency i-level.
Barry Cupples, the chief executive of OMD in Central and Eastern Europe, will take on Cooper's role in Asia.
Cooper joins PHD as it attempts to increase its geographical coverage. Having launched offices in the majority of European markets, its focus is now on opening in markets across Asia.
Omnicom recruited Cooper, who started his media career at Saatchi & Saatchi in London in 1997, to launch OMD from scratch in Asia. He recently took on the wider OMG role, also overseeing PHD in the Asia Pacific region.
Cooper is expected to be based in London and will report to Daryl Simm, the chief executive of Omnicom Media Group.
Cooper was unavailable for comment.