The American-born Ali, who will share the title of joint executive creative director with Jaume, has worked as an art director on television work for Volkswagen's Golf and GTI models and as a creative director on The Guardian account.
Ali has served two stints at DDB since 1999, with the break spent at Ground Zero Advertising in New York.
She has also worked for Clemenger BBDO in Sydney, and Foote Cone & Belding San Francisco and Wieden & Kennedy Portland in the US.
WCRS was forced to extend its search for a joint executive creative director after it was thwarted at the 11th hour in its bid to poach Bartle Bogle Hegarty's creative director, Russell Ramsey, in August last year.
Jaume said: "Without wanting to give her too much to live up to, I think Leslie is a star who will help transform what we do and how we do it. And it will be fun having a creative partner again."
Jaume has been WCRS's sole executive creative director for the past three years. Jaume and Ali will oversee a creative department of 22, including three teams of creative directors.
"We have had an excellent 2003 in terms of our creative product, and not just on one or two star campaigns - there is real strength and depth across our client base," Stephen Woodford, the WCRS chief executive, said. "We want to build on this and in Leslie we have found someone who has the intelligence, passion and energy to partner Leon perfectly. They will lead the agency to achieve even more this year and lead from the front by producing their own work too."
Ali is expected to join the rest of WCRS's management in its management buyout from Havas. The two parties are still negotiating over price, but sources close to the deal say that they expect a result shortly.