His departure is being seen as part of efforts by Tony Granger, the newly arrived executive creative director, to re-energise the creative department with an injection of new blood.
Hanrahan is expected to be replaced by Sally-Ann Dale, who currently shares the deputy head of TV role with Manuela Franzini.
An industry veteran, Hanrahan began his career as an assistant projectionist at the then Boase Massimi Pollitt in 1978 and worked at the Production Company as a producer before his arrival at Charlotte Street in 1990.
He took over as head of TV in March 1995 from Jim Baker, then one of Saatchis' longest-serving staffers and an institution within commercials production.
Hanrahan was Baker's deputy at the time and his promotion came as a surprise.
The job had been expected to go to Martha Greene, then the managing director of Stark Films. She is said to have been rejected because of her close connections with the agency then being established by the ousted Maurice Saatchi.
During his tenure, Hanrahan became actively involved in industry issues and was a leading member of the team that saw off the actors' union Equity in the 1998 dispute involving voiceover artists.
Granger, the former Bozell New York creative chief appointed to succeed Dave Droga at the beginning of the year, is said to see Dale as a rising star responsible for much of the best of the agency's current output.
His decision to change the leadership of the TV department follows closely on other recent changes resulting in the management of the creative department being split three ways. The head of art role held by Nik Studzinski, now in creative command of Publicis, was also scrapped.
Saatchis this week declined to comment on the exit of Hanrahan, who is expected to be retained by the agency on project work.