Beattie McGuinness Bungay had handled the business until July 2006, when the account moved to the specialist retail ad agency Yellow Door.
Since then, the fashion retailer has shied away from producing high-profile brand campaigns.
Fallon's first work for the retailer will include a TV campaign breaking on Sunday during Sky's 24.
The campaign, creative directed by Richard Flintham, was shot by the photography collective Blinkk.
Filmed in black and white, the 60-second TV executions aim to highlight the characteristics of men and women and will be supported by a series of 20-second online films and a print and outdoor campaign.