Talks between Mondelez and the coffee company D.E. Master Blenders began well before investor Nelson Peltz joined Mondelez's board, a source told CNBC, but the billionaire's fingerprints were all over the transaction.
Trian, which owns more than 41 million shares of Mondelez, had been urging the company, which split from Kraft in 2012, to streamline its business and focus mostly on snacks.
According to a source, Mondelez's board wasn't 100% behind such a deal—and even considered other options—before getting behind the joint venture with Peltz's prodding.
Trian had no comment.
Read MoreMondelez, D.E. in coffee deal