Traveling in Erbil, Iraq, on Thursday, Carter told reporters, "This is a mistake I made with respect to email, entirely my mistake, entirely on me."
Carter said he would occasionally use his iPhone that had his personal email on it "to send administrative messages, no classified information" to his immediate staff.
He did so "until a few months ago, which meant I was doing it longer than I should have been. "
“Someone in my position should have known, when the sensitivities on this issue should have known better," said Carter. "I didn't do the right thing.”
He acknowledged that when he became Defense Secretary "there were plenty of people during the time that you're taking office and so forth who explain to you what the rules are about e-mail." He added, "it's not like I didn't have the opportunity to understand what the right thing to do is. I didn't do the right thing. This is entirely on me."
Senator John McCain, chairman of the Senate Armed Services Committee, said in a statement said that he will investigate Carter's use of his personal email "to ensure that sensitive information was not compromised. McCain has requested copies of the emails.
"With all the public attention surrounding the improper use of personal email by other Administration officials, it is hard to believe that Secretary Carter would exercise the same error in judgment," he said in a statement.
Peter Cook, the Pentagon's press secretary, said in a statement that “after reviewing his email practices earlier this year, the Secretary believes that his previous, occasional use of personal email for work-related business, even for routine administrative issues and backed up to his official account, was a mistake.”
“As a result, he stopped such use of his personal email and further limited his use of email altogether. He is confident that his work-related email has been and will continue to be preserved within the federal records system," said Cook.
Cook said that in addition to his official government email, Carter also has a personal email “that he uses to correspond primarily with friends and family.”
“The Secretary does not directly email anyone within the Department or the U.S. government except a very small group of senior advisers, usually his chief of staff,” Cook said. “The Secretary has a secure communications team that handles his classified information and provides it to him as necessary. He takes his responsibilities with regard to classified material very seriously."
Carter’s use of his private email for some official business was first reported by the New York Times.
ABC News' Alexander Mallin contributed to this report.