McGarryBowen's New York flagship is losing its CEO, Bill Borrelle, as he departs for a client-side role.

Mr. Borrelle declined to comment about his new position and did not immediately disclose where he's headed, though it's understood he's not landing at a client of McGarryBowen.

With his departure, the agency is doing away with the CEO title. Jennifer Zimmerman, global chief strategic officer for the agency, will step in as acting president while the agency seeks a replacement via an internal and external search. The incoming exec will be on equal footing with Tim Scott, who serves as president of McGarryBowen's Chicago office.

"We are grateful to Bill for his leadership and significant contribution to the Agency. He has always been an important part of the McgarryBowen family and will remain so even as he moves client-side," said Gordon Bowen.

Mr. Borrelle had been at the agency since 2006 -- prior to its acquisition by Japanese giant Dentsu -- but was promoted in 2009 to lead the shop at the height of its success on the new business circuit; around that time the agency had doubled in size to more than 600 people.

Before that he was focused on digital marketing, working at Digitas and Wunderman in New York. The departure from McGarryBowen marks a return to the client side; Mr. Borrelle spent time at Citi as a marketing director in the early 90s.

For McGarryBowen its the latest in a string of staffers who'd been with the shop since its earliest days exiting, including co-founders John McGarry and Stewart Owen, as well as Mr. McGarry's son (known as J3). It also comes just as the shop is in the middle of a pitch for Liberty Mutual's account.

Mr. Borrelle also joins a growing group of defectors from adland. He's one of several senior agency leaders who decided that, after years of working on Madison Avenue, this summer was the right time to exit the agency world. Bob Lord went to AOL, Peter McGuinness went to Chobani and Anne Bologna went to TripAdvisor.