Phillips was born Holly Michelle Gilliam in Long Beach, California, the second child of Joyce Leon (nee Poole), an accountant, and Gardner Burnett Gilliam, a merchant mariner. She has one older sister. Phillips's mother died of a brain aneurysm when Michelle was five years old. Between the ages of six and twelve, Phillips was raised in Mexico City, Mexico, where her father was studying sociology on the GI Bill at Mexico City College.

After her marriage to John Phillips at age eighteen, the couple relocated to New York City, where they began writing songs together. There, Phillips was a founding member of the Mamas & the Papas, helping to form the vocal group in 1965. She co-wrote some of the band's hits, including California Dreamin', which appears on their debut album If You Can Believe Your Eyes and Ears (1966). Recording of the Mamas and the Papas' second album (eponymously titled The Mamas and the Papas (1966) and sometimes referred to as Cass, John, Michelle, Dennie, whose names appear thus above the band's name on the cover) was interrupted when Michelle Phillips became indiscreet about her affair with Gene Clark of the Byrds. An affair the previous year between Phillips and bandmate Denny Doherty had been forgiven; Doherty and John Phillips had reconciled and ostensibly written I Saw Her Again (1966) about the episode, although they later disagreed about how much Doherty contributed to the song. This time, Phillips was determined to fire his wife. After consulting their attorney and record label, he, Elliot and Doherty served Michelle Phillips with a letter expelling her from the group on June 28, 1966. Michelle was rehired shortly thereafter, when the three original members concluded her replacement Jill Gibson, who was a quick study and well regarded, lacked her predecessor's "stage charisma and grittier edge"; Michelle Phillips was reinstated on August 23, 1966. After Phillip's reinstatement, the band embarked on a brief tour of the East coast, playing a series of precarious shows in Washington, D.C., Baltimore, Maryland, and at Fordham University in New York City. After returning to California and settling in Los Angeles, the group recorded their third album, The Mamas & The Papas Deliver (1967). In June 1967, Phillips performed with the group at the Monterey Pop Festival in Monterey, California, an event organized by John Phillips and Lou Adler. The festival also featured other prominent California-based counterculture musicians and psychedelic rock acts, including Jefferson Airplane, Janis Joplin and Jimi Hendrix. Recounting the experience, Phillips said: "[It was like] a Renaissance Fair. It was convenient for the artists and the audience. Practically everyone had a seat, and if not, people were lining up against the fence, and they could see and hear. Or people were sitting outside, you could hear it outside, too... It was lovely." In August 1967, the band played what would be their final live performance at the Hollywood Bowl. Phillips would go on to record a fourth and final album with the band, The Papas & The Mamas (1968), before going on a hiatus. Michelle and John Phillips, whose marriage was failing at the time, filed for divorce in a Los Angeles County court in May 1969, and the group officially disbanded in 1971 before the release of their final album, People Like Us, which was recorded to fulfil contract obligations with their record label. CANNOTANSWERAnswer this question using a quote from the following article:

What happened in 1965?

There, Phillips was a founding member of the Mamas & the Papas, helping to form the vocal group in 1965.