Katherine Patrick said her father responded to her announcement by giving her a bear hug and saying: "Well, we love you no matter what."
She made her revelation public in an interview with her family published Thursday in Bay Windows, a weekly Boston newspaper aimed at the gay, lesbian, bisexual and transgendered community.
The governor told the paper his family agreed to the interview to make the news public on their own terms.
"The world is such and my job is such that rather than have someone do a 'gotcha' and our giving the misimpression that this wasn't completely natural in our family, then we thought, 'All right, let's just say it and move on,"' he said.
Katherine Patrick, 18, said she told her father and her mother, Diane Patrick, of her sexual orientation on July 3, 2007 - about three weeks after the Legislature rejected an anti-gay-marriage amendment to the state constitution. Patrick, who supports gay marriage, lobbied lawmakers to block the amendment from reaching the ballot.
"He didn't know that I was gay then," Katherine told the paper. "For someone so publicly to fight for something that doesn't even affect him was just like, 'That's my dad,' you know?"
In 2004 Massachusetts became the first state in the nation to legalize gay marriage, after the state's Supreme Judicial Court ruled that it was unconstitutional to deny marriage licenses to same-sex couples. California's high court also has legalized gay marriage, and weddings are set to begin there Monday.
Katherine Patrick did not immediately return a phone call from The Associated Press on Thursday. An aide to the governor said he would make few additional comments other than to say he supports and loves his daughter.
Patrick told the paper the idea that one of his daughters could be gay didn't factor into his decision to support gay marriage.
"I don't think we thought about who they loved - more that they knew what love was and that they would have love in their lives," Patrick said.
Katherine Patrick plans to attend Smith College in the fall and is an intern at MassEquality, a gay-rights advocacy group. She and her father will march in Boston's gay pride parade on Saturday.