Voters overwhelmingly approved a constitutional amendment defining marriage as between a man and a woman, but that ban and others nationwide are facing legal challenges.
Seventeen states allow gay marriage, and federal judges have struck down bans in Michigan, Utah, Texas, Oklahoma and Virginia. A state 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.