Omnicom's PHD has retained its Mercedes-Benz business after a review.
"Every couple of years you want to check outside your universe to make sure you have the best partner for your business," said a Mercedes-Benz USA spokeswoman. "That's the case with PHD."
Omnicom has handled the luxury automaker's media business in the U.S. since 1999, a PHD spokeswoman said.
Mercedes parent Daimler AG spent $317.7 million on U.S. measured media between January and November 2013, a large portion of it on Mercedes, according to Kantar Media.
Last year, Mercedes ended BMW's two-year reign as the No. 1 U.S. luxury auto brand, taking the top spot for the first time in 14 years. And last month, the automaker reported record February sales on the strength of new models, according Ad Age sibling publication Automotive News.
Mercedes' February sales rose 3%, according to the Automotive News Data Center. Rival BMW's sales also rose 3%, while sales for Toyota's Lexus luxury division gained 2%. February sales for General Motors' Cadillac luxury unit dropped 3%.
The automaker is also one of three that will implement Apple's new mobile operating system, CarPlay, this year. The system will let users access functions of their iPhones on the vehicles' interactive dashboard screens.
Mercedes works with a handful of media agencies around the world. Merkley is the creative shop.
Contributing: Michael McCarthy