The album was well-received by contemporary critics, who commended the overall execution and production quality as an improvement over that of the band's debut, although later assessments have faulted its heavy reliance on the template established by its predecessor.

[2][3] Lake ultimately decided to leave, but agreed to sing on the recordings, negotiating to receive King Crimson's PA equipment as payment.

During the recording of the album in early 1970, Fripp, Lake, Tippett and the Giles brothers appeared on Top of the Pops miming "Cat Food".

Fripp persuaded Gordon Haskell to join as singer and bass player, and recruited drummer Andy McCulloch, another Dorset musician moving in the West London progressive rock circle, who had previously been a member of Shy Limbs (alongside Greg Lake, who recommended him to Fripp) and Manfred Mann's Earth Band.

Despite this, Ian McDonald, who composed "The Court of the Crimson King", is not given co-writing credit on this segment of "The Devil's Triangle", only on the opening section, "Merday Morn".

[8][9] The twelve faces in the picture are as follows: Released on 15 May 1970, In the Wake of Poseidon was King Crimson's highest-charting album to date in the UK, reaching number 4.

[11] Robert Christgau rated the album higher than the debut, describing it as "more muddled conceptually than In the Court of the Crimson King" but commenting that "they're not afraid to be harsh, they command a range of styles, and their dynamics jolt rather than sledgehammer".

[13] In his retrospective review, AllMusic's Bruce Eder praised the album, saying that it was better produced than their debut, but he also said that it "doesn't tread enough new ground to precisely rival In the Court of the Crimson King".

[3] All European LPs issued by Island and Polydor have erroneously printed labels that leave off "Peace – A Theme" and list "The Devil's Triangle" and its three movements as four distinct tracks.