Seventeen states allow gay marriage, and federal judges have struck down bans in Michigan, Utah, Texas, Oklahoma and Virginia. An Arkansas judge is expected to rule by Friday in a lawsuit challenging Arkansas' ban.
"I'm going to zealously defend our constitution, but at the same time I think it's important to let people where I stand on the matter," McDaniel told the AP after his speech.
McDaniel said during a question and answer session with editors that there wasn't any single incident that changed his mind about gay marriage.
"It's become more and more difficult for me to accept the idea of anyone being treated as a second class citizen," McDaniel said.
McDaniel announced his support for gay marriage after criticizing U.S. Attorney General Eric Holder for telling his state counterparts in February they weren't obligated to defend laws in their states banning same-sex marriage if the laws discriminate in a way forbidden by the Constitution.
McDaniel said he didn't believe attorneys general should allow their personal views to influence whether they defend a state law.
"I do not take orders from Eric Holder and I'm determined to live up to my obligation, and that includes with regard to our state's definition of marriage," McDaniel said.
McDaniel had voiced support for civil unions when he ran for attorney general in 2006, but said then he believed marriage was between a man and a woman. McDaniel ran briefly for governor but dropped out early last year after admitting to an inappropriate relationship with a Hot Springs attorney.
McDaniel said after his speech that he had considered backing marriage equality during that brief bid for the state's top office.
"Even when I was running for governor, there were times when I was like, 'don't you think it's time that I just say this and be done with it?'" McDaniel said. "It's controversial. There are some people who are going to think it was a great thing that I made this statement while in office, and there are going to be some people who are going to be deeply offended and angry. It really came down to why not today?"
McDaniel said he was not calling on any other statewide officials or candidates to support gay marriage, and said he would not campaign for ending the ban while serving as attorney general.
The head of the group that campaigned for the ban in 2004 said he was surprised by McDaniel's comments but still had faith in McDaniel's office to defend the amendment.
"I'm disappointed because he didn't have to take a position one way or the other. It is disappointing that he has taken this position," said Jerry Cox, president of the Arkansas Family Council. "However, I do not doubt the ability of his staff to adequately defend the Arkansas marriage amendment because I believe the people on his staff are very capable lawyers and I have seen them work very capably on other issues."
The head of the Human Rights Campaign, the largest U.S. lesbian, gay, bisexual and transgender civil rights organization, praised McDaniel for his comments.