The Oscar-winning actress was busy telling the talk-show host all about her upcoming drama/thriller, "Serenity," in which she stars alongside Matthew McConaughey, when a story from filming prompted her to share the personal news.
Hathaway told DeGeneres that McConaughey and his wife, Camila Alves, were the first to arrive on the island of Mauritius, where the movie was shot.
"Matthew and Camila had been there for a while, so we arrived and -- I know Mathew from 'Interstellar,' of course -- and so I got a call from him, 'Hey, you're in the northern part of the island; we're in the southern part. You should come down,'" she explained. "And so we did. And Matthew and Camila took us on an amazing little bar side tour of all the best rums on the island."
"And how was that?" DeGeneres asked.
"I don't remember," Hathaway replied with a bashful smile. "I have no idea. Oh, man. Well -- because -- they're both cool [sighs] and I'm not. And I just can't drink as much as them."
The next day, Hathaway was set to meet with the film's director, Steven Knight. She said she walked in "stumbling" and "with one eye open."
"At the end of it, I said, 'Listen, I've got a confession: I was hungover the entire time,'" she said. "And he just goes, 'Oh, really? I couldn't tell.' And then two days later, we had another meeting, and I showed up and he said, 'Ohhh, now I can.'"
After that, Hathaway decided to take an extended break from alcohol. This happened in October of last year.
When DeGeneres asked the actress if she had parted ways with booze "forever," Hathaway replied, "For 18 years."
"I'm gonna stop drinking while my son's living in my house just because I don't totally love the way I do it," she explained. "And he's getting to an age where he really does need me all the time in the morning. I did one school run one day where I dropped him off at school -- I wasn't driving, but I was hungover -- and that was enough for me. I didn't love that one."