The ruling came more than a year after the appeals court heard arguments in the case.
Proposition 8 backers had asked the 9th Circuit to set aside Walker's ruling on both constitutional grounds and because of the thorny issue of the judge's personal life. It was the first instance of an American jurist's sexual orientation being cited as grounds for overturning a court decision.
Walker publicly revealed he was gay after he retired. However, supporters of the gay marriage ban argued that he had been obliged to previously reveal if he wanted to marry his partner — like the gay couples who sued to overturn the ban.
California voters passed Proposition 8 with 52 percent of the vote in November 2008, five months after the state Supreme Court legalized same-sex marriage by striking down a pair of laws that had limited marriage to a man and a woman.
The ballot measure inserted the one man-one woman provision into the California Constitution, thereby overruling the court's decision. It was the first such ban to take away marriage rights from same-sex couples after they had already secured them and its passage followed the most expensive campaign on a social issue in the nation's history.
The Williams Institute on Sexual Orientation and the Law, a think tank based at the University of California, Los Angeles, has estimated that 18,000 couples tied the knot during the four-month window before Proposition 8 took effect. The California Supreme Court upheld those marriages, but ruled that voters had properly enacted the law.