After the release of the group's third album - Deliver - and their appearance as the closing act of the Monterey International Pop Festival, the group was scheduled to appear in England. The visit was catastrophic for both the group and Cass Elliot (see The Mamas & the Papas main article for further details), and resulted in Cass leaving the group temporarily. The group had completed four tracks for their fourth album - initially titled Springboard - but when the group fracture occurred, progress on the new album stopped completely. Dunhill Records, hoping to keep the group in the public eye while personal matters were sorted out, released a Greatest Hits compilation, entitled Farewell to the First Golden Era, a smash hit at #5 on the charts, and certified gold. Twelve Thirty, one of the completed songs from the fourth album, was included on the album in order to entice record buyers with new material, and simultaneously released as a single. Twelve Thirty would also appear on the now-retitled fourth album, The Papas & The Mamas, when finally released in the fall of 1968. The song peaked at number 20 as a single in the US, but failed to chart in the UK. The group would perform the song on The Ed Sullivan Show on 24 September, 1967, in one of their last televised appearances as a group.

The song was written by John Phillips shortly after the band had relocated to Southern California in 1965. It is often cited as the band's last great single. In a 1968 interview, Phillips cited this arrangement as an example of "well arranged two-part harmony moving in opposite directions".

Jim Ward of Rolling Stone, said "Twelve Thirty" was "the last recording of the self-proclaimed 'Golden Era'", he added, "It's probably the best realized song the group has recorded."

The song was inspired by Laurel Canyon, a neighborhood of Los Angeles, California. In the song, the writer says he used to live in "dark and dirty" New York, where a broken clock on a church steeple was stuck at 12:30. He compares this period to being in "the canyon" (generally accepted to refer to Laurel Canyon, Los Angeles, home to many show business types and where Phillips occasionally resided). A popular interpretation is that girl "groupies" would party into the night at the homes of show business types in the canyon and wander home the next morning, passing the house and engaging the songwriter in conversation as they went. The song fades out during a repeat of the final verse.

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