Although Mustaine's time with Metallica was short-lived, he co-wrote several songs that would appear on the band's debut album, "Kill 'Em All", including "The Four Horsemen" (originally titled "The Mechanix") and "Jump in the Fire." His contributions to these tracks left a lasting mark on Metallica's music.

Mustaine's aggressive playing style and complex riffing were foundational in shaping Metallica's early music. His technical prowess and innovative guitar work on early demos significantly contributed to the band's distinct sound, setting the stage for thrash metal's emergence.

Mustaine's departure from Metallica was a pivotal moment for both parties. While it led to the recruitment of Kirk Hammett, who would become a defining member of the band, Mustaine went on to form Megadeth, another influential band in the thrash metal genre, thus perpetuating his impact on metal music.

The complex relationship between Mustaine and Metallica has been well-documented, marked by public disputes and reconciliations over the years. Despite the early separation, Mustaine's legacy within Metallica and his role in the band's formative years have been recognized and respected by fans and the band members alike.