The 40-year-old city official said prior to Saturday's wedding in Stevensville that the "Life" actress wanted to marry in Montana and he was intent on making her dreams come true, People magazine reported.
"I want it to be everything for her that she ever dreamed of," Newsom said of his 34-year-old bride. "That's why it is Montana. It's her dream, right there on the river. ... I want it to be magical."
Along with the couple's relatives, many top politicians, such as U.S. House Speaker Nancy Pelosi and former San Francisco Mayor Willie Brown, also attended the ceremony.
The marriage is the first for Siebel and the second for Newsom, who earned nationwide fame for prompting a gay marriage debate by ordering a city clerk to issue same-sex marriage certificates.
People said the couple plans on honeymooning in Africa, where Siebel has spent time aiding the Conservation International environmental group.