I think he should as least have been a game show host first.
Ash Carter was appointed to the role of U.S. Secretary of Defense by President Obama. He was in the position from February 17, 2015, until January 20, 2017. Before this role, he was the United States Deputy Secretary of Defense and Under Secretary of Defense for Acquisition, Technology and Logistics.
In general, the U.S. Secretary of Defense position is filled by a civilian who, if having served in the U.S. Armed Forces, has been retired for at least seven years. Congress may grant an exemption to this timeline if desired. In addition, the individual filling the role must understand the United States defense environment and U.S. Armed Forces. They must be able to provide strategic leadership to the department, oversee budgets and make financial decisions, collaborate with military leaders, and advise the president on defense policy and strategy.