Her appointment as the new-business director sees Fallon secure a dedicated new-business person for the first time in its five-year history.
Michael Wall, the managing partner, has assumed responsibility for business development and is putting together a new-business team, led by Weisinger.
"We are planning for the next five years," he said. "Helen is the principal hiring. Her track record is impeccable."
Although Fallon won the pan-European Sony account in 2002, it was absent from significant pitchlists last year.
Weisinger leaves TBWA after a two-year spell as the new-business director.
She joined the agency after quitting Delaney Lund Knox Warren & Partners, which she joined as the new-business development director in September 1998. She was promoted to the head of new business before being appointed to DLKW's board in 2001.
Previously she was an account director on Delta Airlines in Saatchi & Saatchi's New York office, and had a brief stint at RPM3.
"The thing about Fallon is that they are such a great bunch of smart people who are incredibly inspiring," she said.
"It's a great move because it hasn't had a new-business presence, yet what it has achieved is incredible. Once the new-business team is in place, it should help the agency grow even more."