Where Mr. Cella plans to pursue new opportunities is unclear. But his departure from Interpublic doesn't seem to signal his exit from the media industry altogether.
"At Magna Global, I believe my job is done and I am proud of my accomplishments," Mr. Cella said in a statement. "I am looking forward to the next chapter in media which will include the opportunities that a combination of traditional and digital media represent for the future."
An ABC TV veteran, Mr. Cella joined McCann Erickson Worldwide in 1994 as director national broadcast and programming for the Coca-Cola account, and later went on to serve as Magna Global worldwide chairman-CEO and vice chairman at DraftFCB.
"Bill helped to create a new model in the media services world through the establishment of Magna Global, the first agency offering to pool the consolidated media power of our clients to get a better understanding of the marketplace," McCann Worldgroup Chairman-CEO John Dooner said in the statement. "From the early days of his career in advertising sales, to his time at McCann-Erickson, Universal McCann and Magna Global, he has developed a sterling reputation and built a great legacy, as evidenced by his recent induction into the Broadcasting & Cable Hall of Fame."
Mr. Cella's departure comes amid speculation that Interpublic is looking to once again reconfigure its beleaguered media operations.
Media task force
The holding company recently formed a task force, led by Universal McCann Worldwide CEO Nick Brien, to streamline the operations of Universal McCann and sibling Initiative. The formation of the task force stoked rumors that the two units will be merged under Mr. Brien, but Interpublic executives have staunchly denied such a move. Interpublic has previously placed the two media companies under one unit, only to separate them quickly after.
An Interpublic spokesman declined to make Mr. Cella available, and a call to his office wasn't immediately returned.
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Contributing: Megan McIlroy