Rick Savage, Tony Kenning, and Pete Willis, all students at Tapton School in Sheffield, South Yorkshire, formed a band called Atomic Mass in 1977. The band originally consisted of Willis on guitar, Savage on bass guitar after briefly playing guitar, and Kenning on drums. Only 18 at the time, Joe Elliott tried out for the band as a guitarist following a chance meeting with Willis after missing a bus in November 1977.^[9] During his audition it was decided that he was better suited to be the lead singer. The band's initial rehearsals took place at Portland Works, and their first gig was in the dining hall in A Block in Westfield School in Mosborough, Sheffield.^[10]

Elliott proposed the name "Deaf Leopard" which was originally a band name he thought of while designing band posters in art class.^[11][12] At Kenning's suggestion, the spelling was slightly modified in order to make the name seem less like that of a punk band.^[13] In January 1978, Steve Clark joined the band. According to Elliott, he successfully auditioned for the band by playing Lynyrd Skynyrd's "Free Bird" in its entirety.^[14]

In November, just prior to recording sessions for a three-song release known as The Def Leppard E.P., Kenning abruptly left the band; he later formed the band Cairo. He was replaced for those sessions by Frank Noon. By the end of the month, Rick Allen, then only 15 years old, had joined the band as its full-time drummer. Sales of the EP soared after the track "Getcha Rocks Off" was given extensive airtime by renowned BBC Radio 1 DJ John Peel, considered at the time to be a champion of punk rock and new wave music.^[15][16]

Throughout 1979, the band developed a loyal following among British hard rock and heavy metal fans and were considered among the leaders of the new wave of British heavy metal movement.^[17] Their growing popularity led to a record deal with the major label Phonogram/Vertigo (Mercury Records in the US). Def Leppard's original management, MSB, a local duo consisting of Pete Martin and Frank Stuart-Brown, were fired after Martin and Joe Elliott got into a fistfight over an incident on the road. The band approached Peter Mensch of Leber-Krebs management, who had booked them on a tour of the UK supporting AC/DC. Mensch, who admitted that he had had his eye on the band, became their manager.^[18]