Rayner will be responsible for establishing key client relations and providing leadership and strategy in the rollout of multi-country development plans.
He will report to Mauricio Sabogal, worldwide managing director of Initiative.
It is not clear who will replace Rayner as managing director of Ireland, a role he has occupied since 2005.
Before joining Initiative Ireland eight years ago, Rayner was chief executive at DDB's digital division Tribal DDB, which he launched and rolled out internationally.
Sabogal said: "Chris has played an integral role in the development of Initiative's communications strategies and world class planning tools over the last eight years at Initiative Ireland."