The 31-year-old former senior board planner will work alongside Hilde Oorde, the agency's planning director, to manage the growing department, which now comprises 25 staff.
Marco Rimini, JWT's head of strategy, said: "Giles has been chosen for this role because I think he has got the right balance between being good in the boardroom and being able to talk to the people who buy the products."
Hedger will work closely with JWT's executive creative director, Nick Bell, to ensure the agency's planning and creative departments work together efficiently.
Hedger said: "JWT has probably the strongest and most influential planning department in the world. My mission is simple - to remove the 'probably'."