DeCourcy, who joined JWT in May last year, will join TBWA's New York office, where she will report directly to Tom Carroll, worldwide president of the Omnicom-owned company.
It is understood that DeCourcy's new role, which she will take up next month, will be to devise and execute a digital strategy across TBWA's global agency network, which includes Agency.com, Tequila and Integer.
During her time at JWT, DeCourcy worked alongside seven other executive creative directors on accounts for Kleenex, Diageo and HSBC.
Prior to joining JWT, she was chief creative officer at Organic, which she joined in 2000, and devised campaigns for Daimler Chrysler's Dodge Charger and Jeep.
She is also a former director of creative strategy at Toronto-based ICE Integrated Communications & Entertainment, and is a previous chief creative officer at London-based Spafax, where she worked on accounts for British Airways, Virgin Atlantic, Air Canada and Cathay Pacific.
Carroll said: "Her knowledge of the digital landscape, grounded in creativity, makes her an invaluable addition to TBWA."