Based in London, he will work closely with Craig Davis, the recently appointed JWT worldwide chief creative officer, and WPP's Toby Hoare, the chief executive of "team HSBC".
Chaldecott returns to above-the-line advertising after a six-month stint as the boss of a specialist marketing company, SMLXL.
Davis said: "Axel is one of the most innovative and influential thinkers in the business. We are determined to lead change in the industry and Axel is an exemplar of far-sighted creativity."
Chaldecott helped found HHCL in 1987, having raised his creative profile at GGT and WCRS. During his 17 years at HHCL, he launched First Direct and was part of the team behind its famous Tango ads. However, he quit HHCL in 2003, on the eve of the merger with WPP's Red Cell outfit.
Chaldecott said: "Davis has already kick-started a programme of change at JWT. It is in the throes of re-inventing itself and it feels very exciting to be part of this."