Hammett's musical interests eventually drew him into the fledgling thrash metal genre. In 1980, he formed the group Exodus with vocalist Paul Baloff, guitarist Gary Holt, bassist Geoff Andrews, and drummer Tom Hunting. He played on Exodus' 1982 Demo. Exodus was a crucial early player in the Bay Area thrash movement.
Hammett was invited to join Metallica following the dismissal of the band's original lead guitarist Dave Mustaine in 1983. This was prior to the recording of Kill 'Em All. At the time Hammett was taking private guitar lessons from the now-famous Joe Satriani.
Hammett has written and contributed riffs for Metallica songs since the mid-1980s (particularly in the 1990s during the Load era). One of these riffs, used in "Enter Sandman" became one of Metallica's most popular songs. It was the first track and first single on Metallica's self-titled "Black Album" and was ranked 399th on Rolling Stone's list of the greatest songs of all time.
The bridge for "Creeping Death" was originally an Exodus riff that Hammett took with him to Metallica.
Hammett is also known for always having his picking hand taped up. During the course of a full tour, due to constantly palm muting and fast picking, the palm of his hand takes sizable abuse.
Hammett's playing style is noted for his extensive use of the wah-wah pedal in his solos. Hammett says "The wah-wah is an extension of my personality." Hammett's use of the wah-pedal hadn't been featured in recent Metallica solos, but has seen a resurgence in his solos on the newest album, Death Magnetic.
Hammett initially wanted to have guitar solos on Metallica's 2003 album, St. Anger, but drummer Lars Ulrich and producer Bob Rock thought that the solos did not sound right in the songs. He later admitted himself, "We tried to put in solos but they sounded like an afterthought so we left them out".
On April 4, 2009, Kirk, along with Metallica bandmates Lars Ulrich, James Hetfield, and Robert Trujillo and former Metallica bandmates Jason Newsted and the late Cliff Burton, were inducted into the Rock And Roll Hall Of Fame. In 2009 Hammett supplied the foreword to British author Joel McIver's book To Live Is To Die: The Life And Death Of Metallica's Cliff Burton.
In 2005, Hammett played guitar on the Carlos Santana track "Trinity".
In 2006, Hammett voiced himself on The Simpsons ("The Mook, the Chef, the Wife and Her Homer"). He also provided various voices on the Adult Swim show Metalocalypse, including a two fingered fan ("The Curse of Dethklok"), The Queen of Denmark ("Happy Dethday"), and a Finnish barkeep ("Dethtroll"). * He also appeared as a guest in an episode of Space Ghost: Coast to Coast titled "Jacksonville" alongside fellow Metallica member James Hetfield.
In 2010, Hammett will play with Nicolas Abou Chedid on Nico's solo album.
Hammett played guitar on the track "Satan" with Orbital for The Spawn: The Album soundtrack released in July 1997.
After performing a set with Metallica at Bonnaroo in June 2008, Hammett played one song with My Morning Jacket and a couple songs with the annual Superjam collaboration, which also included Les Claypool and members of Gogol Bordello playing primarily Tom Waits songs.
He also appeared on the "Kichigai" E.P. by punk band Septic Death. He played additional lead guitar on the title track.
He appeared as a guest guitarist on K'Naan's "If Rap Gets Jealous" off of the Troubadour album.
He is one of the main characters in Guitar Hero: Metallica along with the rest of the current lineup of Metallica.
Hammett can be seen in the background in Primus' "John the Fisherman" video fishing off of Les Claypool's boat. Claypool and Hammett are childhood friends.
In 2007, ESP Guitar Company announced the release of a 20th Anniversary Limited KH-20 Guitar to celebrate 20 years of relationship with Kirk Hammett (based upon the KH-2 guitar with some modification). Only 41 guitars were in production, with the price at $9,999 USD a piece.
In January 2009, ESP also announced the rare release of Hammett's famous "ouija" guitar, which was very limited and for 2009 only.
Throughout Metallica's career, Hammett has used a range of different amplifiers. For the first two albums, he used Marshall amplifiers and cabinets, with occasional effects. For the recording of 1986's Master of Puppets, he and James Hetfield bought a Mesa/Boogie MarkIIC+ amplifier, and used Mesa/Boogies until he made his move to Randall Amplifiers in 2007.
In September 2007, Randall Amplifiers announced a partnership with Hammett to design a line of signature amps, heads, combos and preamp modules.
Hammett has also been seen using a wide range of different amplifiers in the studio and during live performances.
To avoid problems with pedals being damaged during live performances, Hammett keeps his effect pedals in a rack along with his amplifiers and his tech controls them through a pedalboard sidestage. The pedal controller allows him to change between different effect pedals and amplifiers.
In 2008, Jim Dunlop started working in partnership with Hammett to create a signature Wah-wah pedal, the KH95. It is now available to buy in stores.