Hassall attended Highgate School, where he was classmates with Johnny Borrell of Razorlight .It is reported Borrell was actually part of the band's line up as bass player before Hassall; Another band member was quoted saying he left because he could not keep up with a song they were practising (allegedly 'Horrorshow', a song from the band's Up The Bracket – their first album). Hassall joined a band and ended up playing bass. He played in various bands for a few years, but none of them worked out until he met Pete Doherty. Doherty and Carl Barât had already founded the Libertines, but were in need of a bassist and drummer. It is said that Doherty was intrigued by Hassall not only for his talent but because he had 'proper' equipment. To play the drums on their first recording session, the trio hired Paul Dufour. Hassall was hired to rejoin them on bass when they were signed by Rough Trade just before during December 2001. He continued to play with the band until they disbanded at the end of 2004. The band has since reformed and was a "Special Guest" at the Reading/Leeds Festival on 28 August 2010. According to both Barât and Doherty, The Libertines plan to play live at Hyde Park in London on 5 July 2014 probably with some new material which they may possibly record in the future. John Hassall has also formed a new band The April Rainers  where he lives in Denmark and they are currently recording after making their first live appearance in Copenhagen.