He will be responsible for looking after financial, internal and operational affairs for the advertising agency and its ventures in other disciplines.
CHI's managing partner, Johnny Hornby, said: "Peter will be much more than a finance director. He'll be helping us in a group management role that will see him strategically involved with Naked Inside and Simon Hall's joint venture with us."
Walker added: "CHI is all about big ideas. My role will be to help enable the founders to bring these ideas to life outside of the advertising arena."
Walker has 16 years' experience in the advertising industry. His last permanent role was as the European chief financial officer at BBDO. Walker joined BBDO from Leo Burnett, where he was the chief operating officer, EMEA, for four years.
Since leaving BBDO, he has worked as a consultant to ad agencies and marketing services businesses.