McMillon, 47, has overseen Walmart's fastest-growing division since 2009, keeping up the growth of the world's largest retailer through the global economic downturn. (Agencies)
He has also been in charge when the company has faced damaging allegations of bribery and other misconduct in Mexico, India and other countries, and charges of insensitivity to worker safety in supplier locations like Bangladesh where factory fires and collapses have killed and injured thousands.
A native of Arkansas, the home base of Walmart, McMillon first worked for the company in a summer job when he was 18 and then joined it permanently while in graduate school in 1990.
He held various executive positions, including head of Sam's Club, Walmart's bulk-purchase membership stores.
He follows in the footsteps of Duke, who moved from the international division into the chief executive's job in 2009, expanding Walmart to more than 11,000 stores under various brand names in 27 countries, and a staff of more than two million.
Duke will step down on February 1, 2014. "This leadership change comes at a time of strength and growth at Walmart," said Rob Walton, chairman of the company's board of directors.
"A merchant at heart, Doug has both a long history with our company and a keen sense of where our customers globally are heading next," said Walton, whose family controls the company.
McMillon, 47, has overseen Walmart's fastest-growing division since 2009, keeping up the growth of the world's largest retailer through the global economic downturn.