Dave Mustaine formed Megadeth after leaving Metallica in 1983.
Mustaine had been Metallica’s lead guitarist during the band’s early period, but he was dismissed in April 1983 after disagreements involving substance use, behavior, and musical direction. Soon afterward, he returned to California and began assembling a new band.
Megadeth’s first lineup developed in Los Angeles, with Mustaine serving as guitarist, vocalist, and principal songwriter. The group became one of the leading acts associated with American thrash metal, alongside Metallica, Slayer, and Anthrax.
The timing sometimes causes confusion because Metallica’s debut album, Kill ’Em All, was recorded in 1983 after Mustaine’s departure. Mustaine did not perform on that album, although some songs he co-wrote remained in Metallica’s repertoire.