The San Francisco agency first won the business in 2005 and retained it despite a procurement-mandated review policy.
Hesse earned her first Cannes Lions for a 2009 campaign called "Contemporary Beauty Ideal" that sought to combat eating disorders with classic art.
The French advertising powerhouse and the British government want more control over where their ads run.
More than 6,500 creatives have signed up, with brands like Porsche, Adobe and Heineken showing interest.
Neil Christie becomes COO with Helen Andrews promoted to lead the U.K. team. Jess Monsey was named director of client services in Portland.