She was born in Johnsburg, Illinois — as noted by her husband and musical collaborator Tom Waits, in the song of the same name. Brennan and Waits met in 1980 during when Waits made his acting debute in Paradise Alley and then during the filming of the Francis Ford Coppola film One from the Heart. Waits was composing the original soundtrack for the film, while Brennan worked at the film's studio, American Zoetrope, as a script analyst.
Waits said in a March 31, 1998 radio interview with Chris Douridas on KCRW's Morning Becomes Eclectic:
We met on New Year's Eve at a party in Hollywood. I was leaving the next day. I was moving to New York City and I was never coming back here to the Los Angeles area ever again. That was what I said. But I'd said that before. So we met on New Year's and then I left. I was gone for about four months and then I got a call to do One from the Heart. I came back and I got a little office with a piano in it and I was writing songs and Kathleen was working at Zoetrope. She was a story analyst. Somebody told her to go down and knock on my door and she did and I opened the door and there she was and that was it. That was it for me. Love at first sight. Love at second sight.
They were married on August 10, 1980, reportedly at the Wee Kirk In The Heather Chapel in Las Vegas.
Waits describes his wife as
a remarkable collaborator, and she's a shiksa goddess and a trapeze artist, all of that. She can fix the truck. Expert on the African violet and all that. She's outta this world. I don't know what to say. I'm a lucky man. She has a remarkable imagination. And that's the nation where I live. She's bold, inventive and fearless. That's who you wanna go in the woods with, right? Somebody who finishes your sentences for you.
Brennan is credited as a co-writer and collaborator on numerous songs and projects written since her marriage to Waits and is credited with introducing him to the music of Captain Beefheart. She is generally regarded as the catalyst for Waits' more experimental sound beginning with the album Swordfishtrombones. Waits has said: "She doesn't like the limelight, but she's an incandescent presence on all songs we work on together."
Brennan contributed one of her paintings to the sleeve insert for Waits' album The Black Rider.
Brennan and Waits live in northern California with their three children: daughter Kellesimone (b. 1983) and sons Casey (b. 1985) and Sullivan (b. 1993).
Brennan was also a judge for the 9th and 10th annual The Independent Music Awards to support independent artists' careers.
Brennan's grandfather was from County Kerry in Ireland. Waits and Brennan honeymooned in Tralee, County Kerry, Ireland.