He prepared for defeat, knowing his stance on the issue often has been a lonely one since he opened San Francisco City Hall's doors in 2004 to a same-sex wedding spree, a move that set the stage for Thursday's historic ruling. The same court had ordered him to stop the nuptials.
So when word came in that the justices had overturned the ban, Newsom sat at his desk, surrounded by his closest aides, looking stunned. The mayor was elated
- and vindicated.
Immediately City Hall staffers' cell phones lit up with a text messages declaring, "We Won!" and cheers and applause filled the mayor's office.
"It was an exhilarating feeling, that's the best I can describe it," Newsom said after confirming the justices' ruling with the city attorney.
For Newsom, the ruling was a personal and political triumph on an issue that has come to define him on the national stage. And the timing couldn't be more ideal, as he explores a run for governor or a spot in Congress.
Newsom's actions on Feb. 12, 2004, made headlines around the world, when he issued the city's first gay marriage license to lesbian activists Del Martin and Phyllis Lyon, who had been together 51 years at the time. Newsom's decision to flout state law drew criticism, and not just from conservatives.
"The danger before was that he was principled but out of the mainstream, because not many Democratic candidates shared the same view," said University of California, Berkeley political scientist Bruce Cain.
The rookie mayor, just one month into the job, even got heat from longtime allies, including Dianne Feinstein. The senior California senator said the intense focus on same-sex marriage during a presidential election year contributed to the party's defeat.
Newsom would forever be linked to the image of more than 4,000 same-sex couples exchanging vows at City Hall, a link that threatened to doom his aspirations for higher office.
But he pushed those concerns aside, arguing that same-sex marriage was a fundamental issue of civil rights. The city eventually joined other groups in suing California for violating the state constitutional rights of gays and lesbians
- the issue decided Thursday.
"I've been accused of taking this a little too personally, and I accept that critique," Newsom said.
"I do personally defend Phyllis Lyon and Del Martin when people attack their rights," he said. "I'm about to get married, and no one's criticized my decision-making, but the fact they can criticize their decision making and they've been together 52 years is remarkable."
Now a more seasoned politician, last year handily winning a second term as mayor, Newsom is no less outspoken on the issue.