But Brown said he reached a different conclusion "upon further reflection and a deeper probing into all the aspects of our Constitution.
"It became evident that the Article 1 provision guaranteeing basic liberty, which includes the right to marry, took precedence over the initiative," he said in an interview Friday night. "Based on my duty to defend the law and the entire Constitution, I concluded the court should protect the right to marry even in the face of the 52 percent vote."
Brown, who served as governor from 1975 to 1983, is considering seeking the office again in 2010. After California voters passed Proposition 8 on Nov. 4, Brown said he personally voted against it but would fight to uphold it as the state's top lawyer.
He submitted his brief in one of the three legal challenges to Proposition 8 brought by same-sex marriage supporters. The measure, a constitutional amendment that passed with 52 percent of the vote, overruled the state Supreme Court decision last spring that briefly legalized gay marriage in the nation's most populous state.
Shannon Minter, legal director of the National Center for Lesbian Rights, called the attorney general's change of strategy "a major development."
"The fact that after looking at this he shifted his position and is really bucking convention by not defending Prop. 8 signals very clearly that this proposition can not be defended," Minter said.
The sponsors of Proposition 8 argued for the first time Friday that the court should undo the marriages of the estimated 18,000 same-sex couples who exchanged vows before voters banned gay marriage at the ballot box last month.
The Yes on 8 campaign filed a brief telling the court that because the new law holds that only marriages between a man and a woman are recognized or valid in California, the state can no longer recognize the existing same-sex unions.
"Proposition 8's brevity is matched by its clarity. There are no conditional clauses, exceptions, exemptions or exclusions," reads the brief co-written by Kenneth Starr, dean of Pepperdine University's law school and a former independent counsel who investigated President Bill Clinton.
Both Brown and gay rights groups maintain that the gay marriage ban may not be applied retroactively.
The state Supreme Court could hear arguments in the litigation in March. The measure's backers announced Friday that Starr had signed on as their lead counsel and would argue the cases.