Ryan, previously president of JWT New York, will become president of JWT North America, while Montague, previously co-president and chief creative officer of JWT New York, retains the same job titles with responsibility for North America.
In their expanded roles, Ryan and Montague will continue to oversee JWT New York and JWT Chicago, with the added responsibility of JWT offices in the region including Atlanta, Toronto, JWT CET and JWT Specialized Communications.
The move is consistent with JWT's worldwide management structure in the Asia Pacific, EMEA and Latin American regions.
Bob Jeffrey, the worldwide chief executive of JWT, said: "Ryan and Montague are ambitious and future-forward, yet they have a strong appreciation and understanding of JWT's heritage -- a legacy built on innovation and big brand ideas.
"In this way, they know how to bring the best of JWT's past into the future. I look forward to watching them continue to build the momentum for JWT across North America."
Ryan said: "The momentum in New York is palpable. We have great talent, clients and brands and we look forward to building on that momentum across the region."