Turner, 51, plans to start the Citadel job in September, according to a person familiar with the matter. Having spent 10 years at Microsoft, including overseeing the sales and marketing of Office 365, he is well-versed in managing a large global team and launching technology platforms for wide distribution.
Turner will join Citadel as chief executive officer of the company's securities business, which is centered on a multifaceted trading platform that handles orders for everything from stocks to interest rate swaps. He will also be a vice chairman of the firm.
A one-time cashier at Wal-Mart , Turner was named a corporate officer there at age 29, the retailer's youngest ever at the time. He later became CEO of Wal-Mart's Sam's Club division.
In a statement released Thursday morning, Turner said he was excited to lead an "exemplary team" at Citadel "as we grow this global business." He also thanked Microsoft CEO Satya Nadella and his colleagues there for their leadership and friendship.
Citadel Securities is a huge player in market making. The firm often processes more than a billion equity shares per day in its securities unit; on its website, the company boasts that its share of the U.S. retail investor market is 35 percent.
It also has made a major push into new markets of late. Over the past year, Citadel has begun trading U.S. Treasury bonds, interest rate swaps and custom-designed swaps. Earlier this year, it bought the market-making arm of KCG Holdings on the NYSE floor, giving it the single-biggest footprint of any trader there.