In addition, Chris Cowpe, the group vice-chairman, is leaving DDB after 32 years, as part of wider plans to rejuvenate the agency.
Hackworthy joins from TBWA\Chiat\Day New York, where he has been the chief strategy officer for the past two years. Arriving at the beginning of September, he will assume overall responsibility for the planning department.
Lucy Jameson, the current head of account planning, will report to him.
Together, Hackworthy and Hammersley intend to move the agency into new strategic areas and have formed alliances with two of DDB's network-aligned agencies, OMD Group UK and WWAV Rapp Collins.
These alliances will be overseen by Hackworthy, whose brief is to widen the agency's output beyond television and print advertising.
This is expected to involve a close working alliance with Manning Gottlieb OMD's affiliate strategy agency Good Stuff, launched last year by Ben Hayes and Andrew Stephens with Virgin Mobile as its first client.
Hammersley said: "Most agencies are looking to put some aspect of media or communications planning resource back into their offer, for good reason. We're reluctant, however, to jump into this with a superficial solution which may grab a headline but not substantially change our business."
The WWAV Rapp Collins association will see Jon Goulding, currently the client services director at WWAV, move to DDB, where he will become the director of relationship marketing.
Hammersley said: "This agency invented planning and its rightful place is to be at the forefront of its constant evolution. These moves are designed to ensure that we are."
Hackworthy added: "Working in America has opened my eyes to the extended role branding can play in framing business strategies, shaping organisational behaviour and influencing popular culture. The opportunity to develop some of this thinking with the most respected planning agency in the business is simply not to be turned down."
Cowpe's departure follows that of his long-time colleague Ross Barr earlier this year. Cowpe said: "I've enjoyed 30-plus years at one of the greatest agencies ever, and may even hold a world record as the longest-serving graduate trainee.
"I've done virtually every job possible, worked with the industry's greats and with some wonderful clients. But it's time to move on to new challenges, although I plan nothing more stressful than a short gap year."