Day will report in to Irwin Gotlieb, chief executive of GroupM, and will be based at the company's New York offices.
Day was appointed chief financial officer at GroupM in 2003 and has been with the WPP network for 10 years. He was one of the original management team that launched MindShare globally.
As COO, Day will be responsible for the day-to-day running of GroupM. He will also develop new business opportunities, and look into the possibilities of acquisitions and joint ventures worldwide.
Gotlieb said: "Rupert has been the de-facto COO of GroupM for quite some time now while also carrying the heavy responsibilities of chief financial officer.
"But it's simply too much of a load for one person. This move recognizes the great work he's been doing for us, as well the need for a deeper management structure at GroupM."