"Even though it was allowed, I should have used two accounts. One for personal, one for work-related emails," she told ABC News.
"That was a mistake. I'm sorry about that. I take responsibility," she added.
Clinton, 67, had earlier declined to express remorse for the controversy that has rocked her presidential campaign, inviting criticism from her rivals.
"I do think I could have and should have done a better job answering questions earlier. I really did not perhaps appreciate the need to do that," she said yesterday.
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Clinton argued that she did not violate the law as this was allowed at that time.
"As I said, it was allowed and there was no hiding it. It was totally above board. Everybody in the government I communicated and that was a lot of people, knew I was using a personal email. But I am sorry that it has, you know, raised all of these questions. I do take responsibility for having made what is clearly not the best decision," she said.