But after what she called "a period of careful review and reflection," Whitman has joined the legal fight to overturn the gay marriage ban approved by California voters in 2008.
She is one of dozens of Republicans who have signed a legal filing making a conservative case for Supreme Court justices to strike down the law. The court is scheduled to hear the case next month.
The legal filing was first reported by the New York Times.
"It makes no difference whether the marriage is between a man and woman or a woman and woman," she wrote. "Marriage makes society better."
She added, "Establishing a constitutional right of marriage equality in California will strengthen our nation as a whole."