Grimmer will replace Jeff Hyams, who is leaving Zed to take a new European role running the digital operations of Mediaedge:cia.
Despite not being from a digital or direct background, Grimmer already manages three of Zed's largest accounts: Zurich, MBNA and HP. A managing partner of Optimedia before its merger with Zenith Media, he became a board member of the merged agency. He was previously the planning director and then the client services director at CIA.
Grimmer said: "With a bit of luck and a following wind we should be able to turn Zed into a major agency within a two- or three-year time-frame."
Antony Young, ZenithOptimedia's chief executive, said: "Greg was the strongest candidate, externally and internally. He will bring a heavy client focus and an integrated strategic communication perspective to our proposition."