Peltz had criticized Mondelez in July at the CNBC-Institutional Investor Delivering Alpha Conference—saying it did not generate as much profit as it could, and that it should be acquired by PepsiCo.
Mondelez was spun off from Kraft in 2012.
Peltz was named to the Mondelez board on Tuesday. His Trian Fund Management is the company's fourth-largest shareholder with 2.3 percent of its shares, according to Thomson Reuters.
With the addition of Peltz, Mondelez's board expanded to 12 members, 11 of whom are independent.
Peltz had previously said that he'd like to see Mondelez change its name—arguing the value of the company had gone down because of the name. But now he told CNBC that he's willing to stick with it for now.
Peltz, who is also a major PepsiCo shareholder, told CNBC he'll continue to push the company to break up—arguing that consumer tastes are turning against soft drinks and Pepsi's snack unit is being overshadowed by its underperforming beverage unit.
The idea of a split has been ruled out by PepisCo CEO Indra Nooyi. She told CNBC earlier this month: "These two categories are better together, not just in the United States, but around the world."
—By CNBC's Andrew Ross Sorkin. CNBC's Matthew J. Belvedere and Reuters contributed to this report.