She will leave the health specialist later this year.
Fiddian-Green joined Red Door last July in a board-level role. Her hire was a significant one for the agency as she became the first board-level appointment made from outside its own ranks since the agency launched in 2000.
MD Catherine Warne said: ‘It is sad it didn't work out for either party. Both sides have agreed this will be a positive move for Annabel.'
Warne added: ‘She resigned just before Christmas. She does not have another job to go to.'
Warne said Fiddian-Green, who joined from a director role at Manning Selvage & Lee, was likely to continue working for the agency until June as she has a six-month notice period. Clients with which Fiddian-Green has worked include GlaxoSmithKline (pain), Novo Nordisk (diabetes) and Sanofi Pasteur MSD (vaccines).
Fiddian-Green said: ‘I am still working very closely with Catherine. I really admire Red Door but culturally it is very different from MS&L - I am personally happier working in the culture of a multinational company.'
She added that she was ‘exploring options' at multinational PR agencies based in London.
Warne said the agency planned to announce a ‘very senior' appointment shortly, although the new hire will not be a direct replacement for Fiddian-Green and is for a more junior role.
Fiddian-Green joined Manning Selvage & Lee in 2002. She previously worked at Fleishman-Hillard in its London office.