McMillon, 47, is currently CEO of Walmart International.
The move, which some described as a surprise, comes as Wal-Mart has been struggling with flattish sales growth in the U.S.
"This leadership change comes at a time of strength and growth at Walmart," said Rob Walton, chairman of Walmart's board of directors, in a statement. "The company has the right strategy to serve the changing customer around the world, and Doug has been actively involved in this process. The company has a strong management team to execute that strategy."
Duke, 63, who had been CEO since 2009, will remain as chairman of the company’s executive committee and will stay on as an advisor for one year, Wal-Mart said.
"Mike put in place the building blocks for the next generation Wal-Mart and today the company is stronger, more global and more unified across all our stores, mobile and online," said Walton. "He also reinvigorated the productivity loop and delivered strong financial performance. During his tenure the company made critical investments in talent and technology to expand Walmart to even more customers globally and stepped up its progress on social and environmental issues."
Read more: Wal-Mart Cites Sales Pressures in Q3
McMillon, who is originally from Jonesboro, Ark., began his career at Wal-Mart in 1984 working in a distribution center during the summer. He later joined the company as a store-level worker in 1990.
Much of his career has been in merchandising in the Walmart U.S. division, with experience in food, apparel and general merchandise. He has also held various merchandising positions at Sam’s Club and Walmart International in addition to holding leadership roles in all three operating segments of the company. From 2006 to February 2009, he was president and chief executive officer of the company’s Sam’s Club membership-warehouse division.
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