The latest post means the end of a mammoth underground race to become the first to monetise the account.
"Father of direct marketing" created a trillion-dollar industry.
At CES, the executive chairman told of plans to capitalise on adland's transformation as S4 Capital continues to build out its armory.
It comes after the IPG shop fired Jeremy Perrott in the lead up to Cannes last year.
The company's US president, Sean Lyons, takes the reins. Greenberg will remain active at the company through to 2020.
Luma Partners founder and CEO Terry Kawaja urged marketers to look at the success models of direct-to-consumer brands.
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