[1][2] On 12 July 2015, the song was premiered on the BBC Radio 1 Rock Show with Daniel P. Carter as that day's "Rockest Record", before it was made available for individual purchase online.

During the performance, frontman Oliver Sykes jumped on the table at which Godlike Genius award winners Coldplay were sitting, causing it to collapse.

[16] In line with the loose concept of That's the Spirit,[17] the lyrics for "Happy Song" were written about "making light of a shitty situation", in particular referring to depression.

[19] The track features a prominent children's group vocal section throughout, as well as string and brass instruments in places, built around a number of notable guitar riffs.

[21] Revealing news of the track's release, Rock Sound writer Andy Biddulph described "Happy Song" as "absolutely massive",[3] and in a review of That's the Spirit later dubbed it "epic, thunderous and skyscraping in equal measure".

[22] Gigwise's Andrew Trendell also dubbed the song "massive", describing it as "a monolothic [sic] slab of rock – and a promising sign of things to come".

[25] Some critics claimed that "Happy Song" joined previous single "Drown" in marking a stylistic departure from the band's earlier releases.

Brian Leak of the Alternative Press noted that the track "follows suit with a slightly new direction for BMTH with plenty of melody and next to no screaming from Oli Sykes".

[26] Similarly, Mack of Music Feeds suggested that "the tune will no doubt divide longtime fans already dismayed by what seems to be the metalcore giant’s sudden yellow-brick quest for mainstream crossover appeal".