Peter Foy, who served on the firm's audit committee, said in a press statement he was resigning because of other commitments.
The company had replaced seven directors who also held executive posts within Omnicom in April as part
Mr. Foy, 61, joined Omnicom in 1999. Mr. Foy has held numerous executive positions at other firms. He was chairman of U.K. recruitment firm Whitehead Mann and was on the board at PepsiCo.
He is being replaced by two new outside directors, Michael Henning, retired deputy chairman of Ernst & Young, and Errol Cook, a former managing director and partner of Warburg Pincus & Co. The two will officially join on Dec. 31.
The Omnicom board will now consist of 11 members, including non-executive chair Bruce Crawford, President-CEO John Wren and nine other independent directors.