In December 1965, he saw Eric Clapton with John Mayall's Bluesbreakers at The Refectory, Golders Green, North West London.

[8][9] In April 1968, Kossoff and Kirke teamed up with Paul Rodgers (vocals) and Andy Fraser (bass) to form Free.

Both albums showcased the band's blues- and soul-influenced sound, a style that was in contrast to some of their progressive and heavier counterparts at the time.

[10] The band played the Isle of Wight festival to both audience and critical acclaim, followed by sold-out tours in the United Kingdom, Europe and Japan.

[6] Kossoff co-wrote several Free songs, including "Oh I Wept" and "Mr Big" on the Fire and Water album.

[14] Kossoff's guitar playing was also much in demand for session work and he contributed solos on several albums including: Martha Veléz's Fiends and Angels (1969); Michael Gately's Gately's Cafe (1971) and Mike Vernon's 1971 album Bring It Back Home; Uncle Dog's Old Hat (1972); Jim Capaldi's Oh How We Danced (1972) and Short Cut Draw Blood (1975); The Amazing Blondel's Mulgrave Street (1974).

The late 1990s saw a renewed interest in Kossoff and Blue Soul was re-released, as well as the five-disc Free box set Songs of Yesterday.

In 2011 a selection of early recordings Kossoff made with Black Cat Bones was released on the album Paul's Blues.

[6] Andy Fraser noted "he felt intimidated by those other guitarists, and when people began speaking of him in terms of Clapton and so on, it frightened him.

[18] Kossoff died on a flight from Los Angeles to New York City on 19 March 1976 from a pulmonary embolism after a blood clot in his leg moved to his lung, while touring America with Back Street Crawler.

[21][22] In April 2017 Guitar magazine featured the 'stripped' Gibson Les Paul that Kossoff played at the Isle of Wight Festival in 1970.