John Kahn was born in Memphis, Tennessee. Adopted at birth by the Kahn family, he grew up in Beverly Hills, California. At Beverly Hills High School, he earned a reputation as a precocious musician playing jazz guitar and composing a symphonic piece, "Western Impressions", the first orchestral work by a student to be publicly performed by the school's orchestra under the direction of Robert Holmes.
Kahn first played with Garcia in May 1970 as the bassist of a jazz rock group that coalesced around Garcia and organist Howard Wales during Monday night jam sessions at The Matrix, a San Francisco nightclub of the era; this ensemble would go on to record Hooteroll? (1971; credited to Garcia and Wales). He also played acoustic bass in Old and in the Way, a bluegrasssupergroup that toured from March through May 1973. Along with Kahn, the band featured Garcia on banjo and vocals, future progressive bluegrass luminary David Grisman on mandolin and vocals, Peter Rowan on guitar and vocals, and Vassar Clements on fiddle. From 1970 to 1975, Kahn played in the Merl Saunders and Jerry Garcia Band and was an integral part of later iterations of the Garcia/Saunders collaboration, including Legion of Mary (1974-1975) and Reconstruction (1978-1979); ironically, the latter group was initially intended to serve as a musical outlet for Kahn during Garcia's tours with the Grateful Dead. Kahn was the only founding member of the Jerry Garcia Band (1975-1975) to remain with the group until its dissolution following Garcia's death in 1995. He also played alongside Garcia in the Jerry Garcia Acoustic Band (1987-1988) and served as the guitarist's only accompanist in the Garcia & Kahn acoustic duo that toured intermittently from 1982 to 1989.
Kahn did occasional work for the Grateful Dead, serving as a recording engineer alongside Betty Cantor-Jackson and Bob Matthews for the band's aborted Egypt '78 live album documenting performances at the Giza Plateau in September 1978; selections from these concerts were eventually released as Rocking the Cradle: Egypt 1978 (2008). He also contributed organ and additional production to Shakedown Street (1978) following the departure of nominal producer Lowell George.