Ashton Baldwin Carter served as secretary of Defense in the administration of Barack Obama from February 17, 2015, until Obama left office in January 2017. 

Carter was born on September 24, 1954, in Philadelphia, Pennsylvania. He earned his bachelor's degree from Yale University in 1976 with a double major of physics and medieval history. As a Rhodes Scholar, he attended Oxford University in England and graduated with his doctorate in theoretical physics in 1979. After working on research fellowships, Carter began teaching at Harvard University in 1984 and continued there until 1993. 

From 1993 to 1996, Carter served as the assistant secretary for International Security Policy in the Department of Defense in the administration of Bill Clinton. He became the undersecretary for Acquisitions, Technology, and Logistics at Defense at the beginning of Barack Obama's presidency. In 2011, he became the deputy secretary of Defense, serving as the department's chief operating officer. President Obama nominated Carter for secretary of the Department of Defense on December 5, 2014. The U.S. Senate confirmed him with a vote of 93 to 5 on February 12, 2015.

Carter was awarded the Department of Defense's Public Service Medal five times.