The change is the result of the November elections when Democrats swept the state's top offices.
Herring voted against same-sex marriage eight years ago when he was a state senator, but has changed his mind since, officials told the Post.
His decision follows the Supreme Court's rulings on two same-sex cases in June.
The court ruled denying federal recognition to same-sex marriage couples who were married in states where they are legal is unconstitutional.
But the justices sidestepped whether the state bans on same-sex marriage violate the Constitution.