It follows a chord progression of B♭-Gm-Dm-C.[3] The song received positive reviews from music critics, calling it a great follow-up of their previous collaboration, and noting it as a highlight of the album.

"[4] Kyle Anderson of Entertainment Weekly commented that on "Outside", Harris "finds the perfect balance between [Goulding's] vulnerable warble and the warping synths underneath".

[5] Idolator's Robbie Daw found the song to be "just as pleasant, if not slightly more so, than Calvin and Ellie's previous outing together, 2012's 'I Need Your Love.'

"[6] Carolyn Menyes of Music Times stated, "There's something that works so well about Harris' string effect synth beats and Goulding's whispered ethereal voice.

"[7] Heather Phares of AllMusic felt that the track "doesn't quite recapture the magic of [Harris and Goulding's] previous work but does make the most of her deceptively powerful soprano, this time in a more upbeat setting.

The video ends with Ellie Goulding and Calvin Harris together in the house in the midst of a mid-fight freeze frame, leaving the viewer to question which version was reality.