After rehearsing with Strummer for less than a month, the Clash made their debut on 4 July 1976, supporting the Sex Pistols at the Black Swan in Sheffield. The band apparently wanted to make it on-stage before their rivals in the Damned—another London SS spinoff—made their own scheduled debut two days later. The Clash would not play in front of an audience again for another five weeks.[20][21] Levene was becoming disaffected with his position in the group. At the Black Swan, he approached the Sex Pistols' lead singer, John Lydon (then going by Johnny Rotten), and suggested they form a band together if the Pistols broke up.[22]

The night after their debut, the band members along with most of the Sex Pistols and much of the rest of London's "inner circle" of punks showed up at Dingwalls club to attend a concert by New York's leading punk rock band, the Ramones. Afterward "came the first example of the rivalry-induced squabbling that was to dog the punk scene and undermine any attempts to promote a spirit of unity among the bands involved."[23] Simonon got into a scuffle with J.J. Burnel, the bass player of the Stranglers. A slightly older band, the Stranglers were publicly identified with the punk scene, but were not part of the "inner circle" centred on the Sex Pistols.[23]

With Rhodes insisting that the band not perform live again until they were much tighter, the Clash rehearsed intensely over the following month. Strummer later described how seriously the band devoted itself to forging a distinct identity: "We were almost Stalinist in the way that you had to shed all your friends, or everything that you'd known, or every way that you'd played before."[24] Strummer and Jones shared most of the writing duties—"Joe would give me the words and I would make a song out of them", Jones later said.[25]Sometimes they would meet in the office over their Camden rehearsal studio to collaborate directly.[23] According to a later description of Strummer's, "Bernie [Rhodes] would say, 'An issue, an issue. Don't write about love, write about what's affecting you, what's important."[26]

Strummer took the lead vocals on the majority of songs; in some cases he and Jones shared the lead. Once the band began recording, Jones would rarely have a solo lead on more than one song per album, though he would be responsible for two of the group's biggest hits. On 13 August, the Clash—sporting a paint-spattered "Jackson Pollock" look—played before a small, invitation-only audience in their Camden studio.[27] Among those in attendance was Sounds critic Giovanni Dadamo. His review described the band as a "runaway train ... so powerful, they're the first new group to come along who can really scare the Sex Pistols shitless".[28]

On 29 August, the Clash and Manchester's Buzzcocks opened for the Sex Pistols at The Screen on the Green, the Clash's first public performance since 4 July. The triple bill is seen as pivotal to the British punk scene's crystallisation into a movement,[29] though NME reviewer Charles Shaar Murray wrote, "The Clash are the sort of garage band that should be speedily returned to the garage, preferably with the motor still running".[30] Strummer later credited Murray's comments with inspiring the band's composition "Garageland".[31]

In early September, Levene was fired from the Clash. Strummer would claim that Levene's dwindling interest in the band owed to his supposedly extravagant use of speed, a charge Levene has denied.[32][33] Levene and Lydon would form Public Image Ltd. in 1978. On 21 September, the Clash performed publicly for the first time without Levene at another seminal concert: the 100 Club Punk Special, sharing the bill with the Sex Pistols, Siouxsie and the Banshees and Subway Sect.[34][35][36] Chimes left in late November; he was briefly replaced by Rob Harper as the Clash toured in support of the Sex Pistols during December's Anarchy Tour.[37