Mackenzie Phillips (born November 10, 1959) is an American actress and singer best known for her roles in American Graffiti and as rebellious teenager Julie Cooper Horvath on the sitcom One Day at a Time. She's also known for her role in the Disney Channel science fiction show So Weird.
Born Laura Mackenzie Phillips in Alexandria, Virginia, she is the daughter of John Phillips, singer of The Mamas & the Papas, and his first wife, Susan Adams. She is the sister of Jeffrey Phillips and a half-sister of Tamerlane Phillips, actress Bijou Phillips and singer Chynna Phillips.
Phillips attended Highland Hall Waldorf School in Northridge, California. At age twelve, Phillips formed a band with three of her classmates and was spotted by a casting agent during one of their performances. She was given an audition for a role in the 1973 hit film American Graffiti, which she won.