An American musician and a founding member of the Grateful Dead, with whom he played bass guitar throughout their 30-year career.

Born in 1940 in Berkeley, California, Lesh began his music studies with classical violin, before switching at the age of 14 to jazz trumpet after being exposed to jazz greats such as John Coltrane and Miles Davis. He had a keen interest in avant-garde classical music and free jazz and enrolled at UC Berkeley to become a music major. However, he and a friend Tom Constanten were disappointed by the lack of encouragement for creativity and both dropped out of college and enrolled with the Italian modernist composer Luciano Berio at Mills College (classmates included minimalist composer Steve Reich).

The following summer, Lesh met then-bluegrass banjo player Jerry Garcia and Ron McKernan in Palo Alto. They formed a friendship and eventually in 1965 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 noticed that another group had made a record under the name Warlocks when he found their album at a store. He suggested to the other band members that they change their name.

Lesh had never played bass before joining the band, which meant that he had no preconceived attitudes about the instrument's traditional rhythm section role. Lesh re-defined what the bass could sound like, and in so doing heavily influenced what the Dead sounded like. Instead of being part of the rhythm section, Phil's bass was a low-end guitar, and his improvised interplay with Garcia and Weir made the Dead what it was. 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, along with similarities to fellow San Francisco psychedelic-era bassist Jack Casady).

Lesh, along with Paul McCartney, 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 adopted a more melodic, contrapuntal approach to the instrument. Before 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 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 some of the songs he did contribute, such as New Potato Caboose and Box of Rain, are among the best-loved in the band's repertoire. However, 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.

After the disbanding of the Grateful Dead, Lesh continued to play with its offshoots the Other Ones and The Dead, as well as performing with his own band, Phil Lesh & Friends (one memorable tour paired him with Bob Dylan).

In 1998 Lesh underwent a liver transplant as a result of chronic Hepatitis C infection.

In 2005, Lesh's book Searching for the Sound: My Life with the Grateful Dead was published, which is the only book about the Grateful Dead written by a member of the band.

In 2006, Lesh was diagnosed with prostate cancer (a disease that killed his father) and had an operation to have it removed.