Sloman's first band of note was local Cardiff fave Trapper before he joined Lone Star in time for their 1977 sophomore album, Firing on All Six, supported by a Reading Festival appearance on August 26, 1977, a headline tour, and another tour with Frank Marino & Mahogany Rush later that same year. Lone Star disbanded before a third album could be completed as guitarist Paul Chapman left to take Michael Schenker's spot in  were released in 1994 as part of BBC Radio One Live In Concert.
Sloman and Lone Star drummer Dixie Lee, along with Trapper bassist Pino Paladino, would join forces in Canadian outfit Pulsar with keyboardist Gregg Dechert and former Ian Thomas Band guitarist Dave Cooper. It would prove to be a short lived union as Sloman got the call from the Uriah Heep camp in 1979 to take over for John Lawton. Sloman recorded the controversial Conquest album with the group but departed in 1981, citing "musical differences." An uncredited Sloman also played keyboards on UFO's 1981 album The Wild, the Willing and the Innocent.
After Uriah Heep, he formed the band John Sloman's Badlands and showcased at the Marquee Club but failed to procure a record deal. Badlands featured former Trapper drummer John Munro, Whitesnake's Neil Murray on bass, and guitarist John Sykes prior to his joining Thin Lizzy (and later reuniting with Murray in Whitesnake). Sloman and Murray would both join Gary Moore's band which resulted in the Rockin' Every Night: Live in Japan album.
Sloman went on to record the first of his solo albums, the Todd Rundgren produced Disappearances Can Be Deceptive, released in 1989. It featured contributions from former Trapper and Pulsar bandmates Pino Paladino, John Munro, and Gregg Dechert (who had also spent time with Sloman in Uriah Heep), as well as noted session guitarist Alan Murphy.
Sloman worked on various projects throughout the 1990s, including Souls Unknown with old mates Paladino (who would later serve for a number of years as touring bassist for The Who) and Munro and The Who/Atomic Rooster affiliated guitarist Steve 'Boltz' Bolton. Another project involved Paladino, guitarist Mats Johansson, and drummer Theodore Thunder (Dick Heckstall-Smith, Leo Sayer). Sloman would take on the pseudonym Earl Grangetown for The Beat Poets, a venture into soul music, which also featured former Lone Star bandmate Pete Hurley on bass. Although an album was recorded, it was never released.
Sloman has gone on to produce three more solo albums since the turn of the millennium, Dark Matter (2003) and 13 Storeys (2006), an acoustic affair on which he is credited with playing everything from cello to harmonium, and his latest effort, Reclamation (2010). Toto guitarist Steve Lukather recorded a cover of Sloman's "Jammin' with Jesus" off Dark Matter on his 2008 solo album Ever Changing Times. Lukather had originally cut the song with Jeff Beck years earlier but Beck never put it out.
Sloman guested with NWOBHM veterans Praying Mantis on 2003's The Journey Goes On, providing lead vocals on 3 songs, "Tonight", "Beast Within" and "The Voice." He is also credited on the 2007 soundtrack for Highlander: The Source as the vocalist performing the Queen hits "Princes of the Universe" and "Who Wants to Live Forever" as well as "The Sun Is Gonna Shine", a duet with Tamasin Hardy.