Reid is due to leave at the end of March to spend time at his homes in Vancouver, Hong Kong and London. Scott Hollingsworth and Ben Barnes will share many of his responsibilities.
Hollingsworth, currently the European president of FCB Worldwide, will continue to lead Europe and will remain based in London. However, he will take on additional responsibilities for Latin America and the Middle Eastern regions.
Barnes will continue to lead Asia-Pacific from his office in Hong Kong.
He adds responsibility for the Africa region and will work closely with Neil van der Weele, the group chief executive officer, FCB South Africa.
Both Hollingsworth and Barnes will now report directly to Brendan Ryan, the worldwide chairman of FCB.
Reid joined FCB from Ogilvy & Mather in 1994. He had spent 22 years at O&M in various roles around the world including the regional director for Asia-Pacific and for Europe, before becoming the chief operating officer for O&M Worldwide.
His time at FCB, where he was in charge of all non-US operations, has seen many changes including the end of its working relationship with Publicis, the creation of True North, the acquisition of Bozell and FCB's own acquisition by Interpublic.
Ryan said: "FCB around the world was very different a decade ago than it is now. In truth, in 1994, FCB actually was represented by Publicis in Europe, was tiny in Asia and was very under-developed across Latin America.
"Largely as a result of Harry's efforts, FCB in the past ten years made almost 40 acquisitions, ranging from huge agencies such as ULKA in India to Giovanni in Brazil, to smaller operations in Poland and Korea."