Sleepytime Gorilla Museum formed in 1999 in Oakland, California, USA. The band was founded by Nils Frykdahl (guitars, flutes, vocals) and Dan Rathbun (bass, log, trombone, lute, vocals) of the band Idiot Flesh, with Carla Kihlstedt (violins, percussion guitar, autoharp, organ, bulbul, vocals) of Tin Hat Trio, 2 Foot Yard and The Book of Knots, after the three worked together in Charming Hostess. Matthias Bossi (drums, glockenspiel, xylophone, vocals) and Michael Mellender (percussion, assorted melodic instruments, vocals) both joined in 2004. In 2025, Matthias passed the drum throne to Wes Anderson — a longtime collaborator and drummer for Idiot Flesh and Charming Hostess. Previous SGM members include founding drummer David Shamrock, percussionist Moe! Staiano, and drummer Frank Grau.

The band tour the U.S. frequently in a renovated vintage Green Tortoise bus, and their live performances have featured puppet shows, pseudo-scientific scholarly presentations, and performances by members of the Butoh group inkBoat. SGM's music has been categorized as anything from avant-prog to art rock to experimental metal to rio (Rock In Opposition). Many reviewers have remarked that the group is altogether unclassifiable.

The band uses many homemade devices as instruments, such as the viking row-boat and the spring-nail guitar. Rathbun plays a stringed bass instrument of his own construction called The Log which uses piano strings and is perhaps 7 feet long; it is played with two sticks: one in the left hand generally used as a fret, another in the right hand striking the strings. Mellender's percussion instruments consist of restaurant kitchen equipment, trash can lids, and other found metal objects, in addition to traditional percussion instruments.