Granger, who was brought in to replace Dave Droga last year, embarked on a restructure of the creative department that resulted in several senior departures.
In June 2003, Nik Studzinski, the agency's head of art, left with his creative partner, Gavin Kellett, while Howard Willmott and Duncan Marshall quit in April 2003.
Last August, Granger appointed Andy Gulliman from Bartle Bogle Hegarty to replace the outgoing head of TV, Mark Hanrahan, who had spent 13 years at the agency.
Granger is set to divide his time between the London and the New York offices until the Saatchis worldwide creative director, Bob Isherwood, can appoint his successor.
Granger's appointment as the New York office's most senior creative will see him work alongside the newly appointed chief executive, Mary Baglivo, who is leaving her position as the president of Arnold Worldwide.
Kevin Dundas, the Saatchis chief executive, said: "It is a wonderful opportunity for Tony and where he ultimately planned to be. We are thrilled for him."
Granger said: "It has always been part of the plan to return to New York. I was sent to the London office to work with Kevin to pull the agency back on track, to help restructure it and to get the awards back on track, and this is what we have done - I am leaving the agency in great shape."