During these days he meets the guitarist of Anvil Chorus, Kurdt Vanderhoof, from San Francisco, who convinces him to join the band, but Lars wanted to stay in Los Angeles.

That same year, Lars meets James Hetfield in Downey, California and they form the band Metallica. He became known as a pioneer of the fast thrash metal rhythms heard in many Metallica songs such as “Metal Militia” from Kill 'Em All, “Fight Fire With Fire” from Ride the Lightning, “Dyers Eve” from ...And Justice for All, and “Battery” from Master of Puppets. Lars Ulrich's influence is quite large, this is due to both the popularity of the band and his playing technique, such as the use of two bass drums in the song “One” (...And Justice for All) and at the beginning of the song “Enter Sandman” (Metallica/“The Black Album”). Discussing Metallica, in October 2007, Evile frontman Matt Drake stated that on “The Black Album” the drums create the best drum sound he had ever heard.