Nearly a year after announcing the deal, Omnicom has officially closed its $13.5 billion acquisition of Interpublic Group, creating the world’s largest advertising and marketing holding company by revenue and billings.

Under the all-stock agreement, Omnicom shareholders will own 60.6% of the combined company’s stock, and IPG shareholders 39.4%. Combined, the companies posted an estimated $26 billion in annual revenue in 2024.

John Wren will remain chairman and CEO of the company, while Phil Angelastro stays on as evp and CFO, and former IPG CEO Philippe Krakowsky and Daryl Simm will serve as co-presidents and COOs.

“This is a defining moment for our company and our industry,” said John Wren, Chairman and CEO of Omnicom, in a statement. “With the completion of the deal, Omnicom is setting a new standard for modern marketing and sales leadership–creating stronger brands, delivering superior business outcomes, and driving sustainable growth. We’re excited about this next chapter. I want to thank our people, clients, and shareholders for the trust they have placed in us.”

First announced in December 2024, the acquisition cleared its last regulatory hurdle earlier this week when the European Commission granted approval. The EU’s decision came two months after the U.S. Federal Trade Commission cleared way for the deal to close in September, with a consent decree prohibiting Omnicom from directing advertiser spend based on political or ideological preferences.

The combination of two rival holdcos will remap the global agency landscape. The closest historical precedent was the proposed 2013 merger between Publicis and Omnicom, which ultimately collapsed over leadership disagreements.

By acquiring IPG, Omnicom is scaling up on data and technology to better serve its enterprise clients across the marketing funnel and compete in a consolidating media landscape. It’s a shift has already pushed rival holding companies to simplify and invest in data and tech over the past decade.

Omnicom has projected $750 million in cost synergies as a result of the combination. To get there, so far, IPG has eliminated 3,200 jobs since January, while Omnicom culled 3,000 jobs at the end of last year.

CEO John Wren has emphasized that Omnicom is working to keep client-facing teams intact, while back-office roles are more likely to be affected.

As the new Omnicom takes shape, it will have to carefully navigate new client conflicts. It now works with fierce rivals across categories, such as AT&T and T-Mobile in telco, or State Farm and GEICO in insurance.

Careful change management will also be key, as it is widely expected that not all of the agencies will survive under their new parent. Last month, reports circulated that Omnicom will eliminate its global DDB network; in November, global CEO Alex Lubar Left the agency without plans to backfill his role.