[1] In July 2012, it was officially confirmed that the group had reformed under the name Mutya Keisha Siobhan and were writing songs for a new album under Polydor.

[3] "Flatline" was written by Keisha Buchanan, Mutya Buena and Siobhán Donaghy, alongside British musician Dev Hynes.

[5] The group pursued a Coldplay-influenced sound for the song's chorus, however after realizing that their voices were not "manly enough",[7] Hynes contacted band Spector member Fred Macpherson to sing backing vocals for the track.

[14] Having a length of three minutes and fifty-one seconds (3:51), "Flatline" is a pop rock song with R&B influences in the vocals, and was produced by Dev Hynes.

[16][17] It commences with a "noodly" guitar riff,[17] set against a 90s-inspired piano-driven R&B melodies and "bubbling"[15] 80s pop beats, which is reminiscent of the production found in American recording artist Solange Knowles' extended play True—which, much like "Flatline", was produced entirely by Hynes.

Siobhán Donaghy sings the first verse of the track, being followed by Keisha Buchanan, who gives her vocals during the pre-chorus, in which she references the playing cards King and Queen.

[15] After the chorus, the second verse is sung by Mutya Buena, and the song eventually enters a crescendo, as the backing vocals get louder and the drums stronger.

[19] Robert Copsey of Digital Spy was extremely positive about the song, awarding it five stars out of five, and commented: "[It] isn't the sort of smack around the face you'd normally expect but rather one that draws you in slowly but surely before swallowing you whole.

"[20] Writing for The Guardian, Michael Cragg said there was "something indescribably captivating" about the song, and likened it to the group's debut single "Overload"—as the Sugababes.

[21] Laura Snapes from Pitchfork Media was enthusiastic about Hynes' production, classifying it as "immaculate", and highlighted the group's vocal harmonies.

[16] Robbie Daw from the music website Idolator was in sync with the aforementioned comments, calling it a "synthy, handclappy, glorious pop gem that comes just in time for the summer".

[23] Carl Smith of Sugarscape.com deemed "Flatline" "a bit amazing" and believed that the artists' harmonies could rival the group Little Mix, while noting that the song did not follow any current musical trend.

[24] Bradley Stern from MuuMuse opined that the song had "revitalized the very essence" of the Sugababes, named it one of the best of 2013 and commented that it sounded like a newly recorded version of One Touch.

[15] Much like previous critics, Michelle Kambasha of DIY highlighted the singers' harmonies and noted that Hynes had "inject[ed] his solid-gold formula" to the song's production.

[25] Drowned in Sound's writing staff picked "Flatline" included it in their list of 2013's best songs on number sixteenth,[26] as did Edge on the Net, that put it on seventh place.

[34] During their headline set at Scala, a nightclub in London, held on 1 August 2013, Mutya Keisha Siobhan played various Sugababes-era hits alongside new album songs, and closed the performance with a medley that mixed "Flatline" with select verses of "Push the Button".

[40] The visual begins with a shot of the three singers walking down a street in Venice Beach, followed by them getting ice cream and being on top of a motorcycle.