Wilsher, who joined Fallon in 2000 as an account manager, fills the position left vacant by the departure of Chris Hirst to become the managing director of Grey Worldwide London.
As an account director, the former Campaign Face to Watch ran the Green & Black's account and was part of the team handling the Sony Europe business, which Fallon won from Saatchi & Saatchi last year.
"This is an easy decision for us. Karina is a great ambassador for our brand and our people," Michael Wall, a partner at the agency, said.
Wilsher began her career with Lowe Lintas in 1998 as an account manager.
She said: "The role gives me an opportunity to look at the department as a whole to improve and strengthen it. There have been a lot of changes at Fallon since I started; both the agency and the clients are considerably larger."