The day after Metallica finished the North American leg of the Poor Re-Touring Me Tour in San Diego's Coors Amphitheatre, they hit the studio to start recording a new album of cover versions. As Lars Ulrich explained, the band wanted to do something different after "three pretty serious albums in a row, starting with the Black album and then Load and ReLoad", and the process would be easier by working with covers, specially as the band had a tradition of taking other people's songs and "turn them into something very Metallica, different from what the original artist did". Given the band had recorded many covers that were spread across various releases, such as B-sides of their singles and the 1987 EP The $5.98 E.P.: Garage Days Re-Revisited, the band would "put them all in a nice little packaging for easy listening" along with the newly recorded cover versions, chosen through a group decision. Only one of the eleven songs in the "New Recordings '98" disk was not done in the three-week sessions, a version of Lynyrd Skynyrd's "Tuesday's Gone" the band recorded for a radio broadcast along with friends such as Les Claypool, John Popper and Gary Rossington.
The cover for Garage Inc. had a Ross Halfin photograph of Metallica dressed as mechanics. The band wanted the booklet to hold a detailed account of the contents of the project, and designer Andy Airfix was allowed to search through Ulrich's catalogue of Metallica memorabilia in San Francisco to create a 32-page booklet. Airfix also did the back cover, where the front of Garage Days Re-Revisited was modified with headshots of Metallica in 1998 and the track list written on tracing paper.
Rolling Stone (12/10/98, print edition, p. 122) – 4 Stars (out of 5) – "Gloriously hard as the album is, you can't miss Metallica's good natured side coming through."
Entertainment Weekly (12/18/98, p. 84) – "We'll have to wait until Metallica's next 'proper' album to find out if this trip to the garage recharges their batteries. Still, all things considered, Garage Inc. is an intermittently exhilarating joyride." – Rating: B−
CMJ (12/21/98, p. 29) – "Those who still relate to the adolescent angst of the 'Metallicas' earliest days will find plenty to like on Garage Inc."
In 2005, the album was ranked number 500 in Rock Hard magazine's book of The 500 Greatest Rock & Metal Albums of All Time.
"Last Caress/Green Hell" contains a parody of Iron Maiden's song "Run to the Hills" at the end. Iron Maiden responded to this on a B-side cover of the Montrose song titled "Space Station No. 5"
"Am I Evil?" and "Blitzkrieg" were initially B-sides to the International "Creeping Death" single and were also bonus tracks on the 1988 Elektra reissue of their first album, Kill 'Em All; they have since been removed from that album (together these covers were known as Garage Days Revisited).
In the U.S., Garage Inc. sold 426,500 units in the first week of release, making Metallica's fourth straight debut surpassing 400,000 copies. Still, the million-plus record breaking debut of Garth Brooks' Double Live made Garage Inc. land only at second on the Billboard 200. As of 2003 the album has sold over 2.5 million copies and has been certified 5× Platinum.