In addition, partner and global chief operating officer Dave Luhr will take the role of president. Co-founder and current global president Dan Wieden will become chairman of the global management team.
To replace Fitzloff, W&K Portland creative director Joe Staples has been promoted to the role of the executive creative director of W&K Portland.
DeCourcy founded Socialistic, a social media content and design shop, in 2010 as chief creative officer and chief executive. While there, she won accounts such as Red Bull and Fast Company, and led work for Showtime, Coke and GE.
Fitzloff joined W&K Portland in 1999 as a copywriter. As creative director on Old Spice, he resurrected the brand and in 2008 he was appointed to the Portland management team as executive creative director.
Luhr has been with W&K since 1986, becoming managing director of the Portland office in 1992 and then partner.
Wieden said: "I don’t think we’ve ever had a chairman, but now I am one – albeit an active one.
"Between Dave and Mark, we have forty years of proven W&K experience, respect and leadership. This level of talent and experience gave me the confidence to search outside the agency to find a third global management team member with digital expertise.
"Colleen brings an energy and a knowledge base that is second to none. I am convinced that our next thirty years are going to be pretty damn exciting."