The financial services company was Fallon's largest account, and the departure is a blow to the network's Minneapolis headquarters.
The shift to Publicis is understood to follow an intervention from Publicis Groupe's chairman, Maurice Levy, to ensure the business stayed within the group.
Fallon's grip on the business was questioned last year when the company looked outside of Fallon for an £8 million pan-European brief to launch a new product called Equity First, a business-to-consumer investment product.
In December last year, Fallon Worldwide appointed Jeremy Bowles, the former managing director of Lowe, to run the CitiBank account.
Most of the creative originated from Fallon Minneapolis. The last work created by Fallon's London office appeared in 2003 and featured a break-dancing dog promoting the bank's "live richly" tagline.