After passing his audition for Frank Zappa's band, Warren, at the age of 22, was officially hired on for the Human Jukebox tour. Another member of the band was drummer Vinnie Colaiuta, who had replaced Terry Bozzio in early 1978. Warren's main job would be to play rhythm guitar for Frank to solo over, and on the tour there were also two other guitarists, Ike Willis and Denny Walley. However, Zappa was impressed enough with Warren's talent that Warren was allowed to function as a musical director of sorts, helping other band members learn parts to songs they had never played before.
The
tour was a stint in Europe, and Zappa ran a tight ship. If the band wasn't playing
right, he might stop the show and re-start them, demanding perfection, or he might
begin to spontaneously improvise, yet the band was expected to keep up and improvise
right along with him. Some of Warren's work on this tour has been released on
the Zappa albums Tinseltown Rebellion, Shut Up & Play Yer Guitar and You Can't
Do That On Stage Anymore (vol. 1, 4, and 6).
In the summer of 1979, the band returned to Los Angeles, and Zappa began work on the Joe's Garage albums. Warren played all of the rhythm guitar and did some vocals, on top of being immortalized in several song lyrics. According to The Real Frank Zappa Book, Diva Zappa's impending birth and some trouble with some of the band members caused Zappa to cancel the fall 1979 U.S. tour and fire most of the band. Those remaining, including Warren, were kept on salary but not working, and Warren began to experiment with songwriting, with his new friend, fellow Joe's Garage performer Dale Bozzio.
Much more detail for those interested here.