Judith Nathan previously had spoken of two marriages. She has now confirmed a third.
"Something I will share with you is that ... Rudy and I have both been married three times," Nathan, 52, told the New York Post.
In her first, previously undisclosed marriage, Nathan, then Judith Stish and a 20-year-old, married Jeffrey Ross in Las Vegas in 1974. The marriage ended in divorce in 1979.
Five days after the divorce, she married Bruce Nathan, a wallpaper salesman. That marriage ended in a bitter divorce in the early 1990s.
Giuliani, 62, married Nathan following a bitter divorce from his second wife, Donna Hanover, in 2000. Nathan said Giuliani had known of her first marriage early on in their relationship.
Nathan has assumed an important role in Giuliani's thriving presidential campaign, but a Giuliani strategist said her disclosure would not damage Giuliani's candidacy.
"It has no political significance and is something that all the people close to her knew," the strategist told the New York Daily News.
But many political observers say Giuliani's marital track record is a negative, especially among Christian conservatives.
"I guess birds of a feather do flock together," University of Virginia political science Prof. Larry Sabato told the Daily News. "This is right off of Match.com."