Mr. Miller is said to have battled with Euro RSCG over the lack of contracts and formal bonuses for top Tatham executives, and to have complained that Tatham wasn't being provided with adequate funding for new hires, particularly at the top creative level.
Mr. Miller was one of three managing partners at Tatham, along with Ralph Rydholm, chairman-CEO and chief creative; and Mary Baglivo, client service director.
Ms. Baglivo said it's unclear whether Mr. Miller will be replaced. He will work as a consultant to Tatham through the end of November.
Mr. Rydholm said the resignation stemmed from Mr. Miller's desire for a promotion and not from any problems between Tatham and Euro RSCG.
"We have a very good relationship with Euro RSCG," he told Crain's Chicago Business.
Executives at Euro RSCG couldn't be reached for comment on the dispute.
Even though the agency holding company has a presence in many major global markets, including Chicago, Paris and New York (with Messner Vetere Berger McNamee Schmetterer/Euro RSCG), Euro RSCG has no presence in some key areas, including Latin America. Management is in the midst of trying to assemble a network there through startups and acquisitions.
Tatham clients U.S. Robotics and Hardee's Food Systems were among those expressing concern in recent weeks over Mr. Miller's possible departure, agency insiders said. Mr. Miller played a crucial role in bringing both clients to Tatham. The $80 million Hardee's win was Tatham's biggest in recent years. In all, the agency has $400 million in billings.
"There is no question Ian was a cornerstone of our account," said Sharon Hamilton, marketing communications manager at U.S. Robotics, in an interview two weeks ago.
U.S. Robotics awarded its $5 million Sportster modem brand to Tatham this summer.
Brown & Williamson Tobacco Co. said Mr. Miller's departure "will not affect Brown & Williamson's relationship with the agency." This year Brown awarded Tatham $20 million in new business for the Misty and Capri cigarette brands.
Mr. Miller's departure comes as Tatham lobbies for additional assignments from Procter & Gamble Co., said to be looking for a fifth global agency. Tatham currently handles P&G brands including Vidal Sassoon, Old Spice, Mr. Clean and Head & Shoulders. Ms. Baglivo is known to be well-loved by P&G executives.
Rumors also persist that former N.W. Ayer & Partners Chairman Steve Dworin is in talks with Euro RSCG about a worldwide post.
Joseph B. Cahill of Crain's Chicago Business contributed to this story.