Lesh started out as a trumpet player with a keen interest in avant-garde classical music and free jazz; he also studied under the Italian modernist Luciano Berio (a classmate was the minimalist composer Steve Reich). While still a college student he met then-bluegrass banjo player Jerry Garcia. They formed a friendship and eventually Lesh was talked into becoming the bass guitarist for Garcia's new rock group, then known as the Warlocks. He joined them for their third or fourth gig (memories vary) and stayed until the end.

Lesh had never played bass before joining the band, which meant he learned "on the job", but it also meant he had no preconceived attitudes about the instrument's traditional "rhythm section" role. Indeed, he has said that his playing style was influenced more by Bach counterpoint than by rock or soul bass players (although one can also hear the fluidity and power of a jazz bassist such as Charles Mingus or Jimmy Garrison in Lesh's work).

Lesh, along with Jack Bruce, John Entwistle, and Jack Casady, was an innovator in the new role that the electric bass developed during the mid-1960s. These players adjusted the bass so that it was louder and had a plush, pervasive timbre. Previous to this, bass players in rock had generally played a conventional timekeeping role within the beat of the song, and within (or underpinning) the song's harmonic or chord structure. While not entirely abandoning these aspects, Lesh took his own improvised excursions during a song or instrumental. This was a characteristic aspect of the so-called San Francisco Sound in the new rock music. In a great Dead jam, Lesh's bass is, in essence, as much a lead instrument as Garcia's guitar.

Lesh was not a prolific composer or singer with the Grateful Dead, although the songs he did contribute—"Box of Rain", "Unbroken Chain", "New Potato Caboose", "Childhood's End", etc.—are among the best-loved in the band's repertoire. His interest in avant-garde music was a crucial influence on the Dead, pushing them into new territory, and he was an essential part of the group and its mystique, best summed-up in the Deadhead truism: "If Phil's on, the band's on".

After the disbanding of the Grateful Dead, Lesh continued to play with its offshoots The Other Ones and The Dead, as well as performing as Phil Lesh and Friends (one memorable tour paired him with Bob Dylan) and running the charitable Unbroken Chain Foundation.

Lesh and wife Jill have two sons.

In 1998 Phil underwent a liver transplant as a result of chronic Hepatitis C infection; since then, he has become an outspoken advocate for organ donor programs. Since then, on Phil & Friend's tours, and during The Dead's tours, he regularly gave a speech (dubbed a "donor rap" on the band's live CD's from each show of the tour) encouraging Deadheads to be organ donors.

In April, 2005, Phil's book Searching for the Sound: My Life with the Grateful Dead (ISBN 0316009989) was published. To date, this is the only book about the Grateful Dead written by a member of the Grateful Dead.

In June of 2006, Lesh will be appearing at the Bonnaroo music festival.