Before joining Saatchis, Gannan spent three years at the Sydney-based digital agency NetX. He has also worked at Ogilvy Interactive, BBDO Interactive and Grey Interactive in Toronto.
Gannan has won numerous awards for his work on clients including 3, IBM, Philips, Unilever and Motorola.
The Saatchis Sydney chief executive officer, Simone Bartley, said: "Sean's hiring is both exciting and pivotal to us delivering truly media-neutral ideas. He is passionate, talented and very, very Saatchi. We are looking forward to immersing him in our clients' business."