Cox is the only surviving musician to have regularly played with Hendrix: first when both were in the Army, then in 1969 with the experimental group that backed Hendrix at Woodstock (informally referred to as "Gypsy Sun and Rainbows"), followed by the trio with drummer Buddy Miles that recorded the live Band of Gypsys album, and, lastly, The Cry of Love Tour trio with Mitch Mitchell back on drums.

Born in Wheeling, West Virginia, Billy Cox was raised in Pittsburgh, Pennsylvania, and attended Schenley High School.

[3] Hendrix eventually left Nashville, playing all over the U.S. in the backing groups of several famous artists (most notably Little Richard and the Isley Brothers) until he was "discovered" by Chas Chandler in New York City's Greenwich Village.

Following the break-up of the Experience, Cox performed with Hendrix's experimental group, informally called Gypsy Sun and Rainbows.

Hendrix then formed another short-lived group with Cox and Buddy Miles, that recorded Band of Gypsys, the live album that he owed former manager Ed Chalpin as part of a legal settlement.

Cox contributed most of the bass parts on the first group of posthumously released Hendrix albums, including The Cry of Love (1971), Rainbow Bridge (1971), War Heroes (1972), and Loose Ends (1974).

Cox has also performed some dates along with Mitchell and guitarist Gary Serkin with a Hendrix-tribute outfit called the Gypsy Sun Experience.

In 2004, Miles reunited yet again with Cox of the Band of Gypsys to re-record songs from the original live album of 1970 with guitarists Eric Gales, Kenny Olsen, Sheldon Reynolds, Andy Aledort and Gary Serkin.

On June 23, 2019, Cox was inducted into the Rhythm and Blues Music Hall of Fame along with his bandmates, Hendrix and Miles for their groundbreaking work as the Band of Gypsys.

Cox was on hand to accept the award in Detroit, Michigan, and performed with guitarist Dani Robinson and drummer Richie Monica.