She will take up the role of president on January 1 2004, overseeing an office that has billings of more than $1.4bn (£830m) and 500 employees, with clients including Pfizer, Unilever and Rolex.
Jeffrey said: "Rosemarie Ryan has the creativity, credentials and charisma to take the transformation of JWT New York to the next level.
"She's known for championing great brand-building ideas; forging highly successful partnerships with her clients and providing dynamic leadership that brings out the best in her team."
Ryan, 40, was president of Kirshenbaum Bond & Partners since 1995, but she originally joined the agency in 1991, before moving to Angotti, Thomas, Hedge in New York.
She began her career in account planning at BBDO Worldwide in London, before moving to the US in 1989, joining TBWA\Chiat\Day in New York.
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