Sigrid performed "Mirror" several times in 2021, including at the Reading and Leeds Festival and Jingle Bell Ball.

After its supporting concert tour concluded in December 2019, she chose to take a break from releasing music in the following year to focus on the development of her second studio album.

[4][10][11] The song was written by Sigrid alongside Caroline Ailin, Emily Warren and Sylvester Sivertsen, while produced by the latter under the pseudonym "Sly".

[10] Writing for Stereogum, Chris DeVille asserted that the song is "driven by a powerful bass groove, bolstered by contagious bursts of house piano, and accented by a string section".

[11] Jason Lipshutz, for Billboard, asserted that during the chorus, Sigrid "gathers up the shards of her confidence" with "strings fluttering around her" and declares, "I love who I see, looking at me, in the mirror!

"[4] According to Thomas Smith of NME, the chorus's "buoyant, direct message" is "under-pinned by slick beats and party-starting piano stabs".

[4] Carley affirmed that the song's bridge is a "homage to disco legends of yesteryear (with shades of Giorgio Moroder and Chic)" and lauded Sigrid's vocals.

[11] BBC Music reporter Mark Savage named it "a blossoming disco anthem about conquering loneliness and emptiness".

[10] Smith stated that the song is "a celebration of the incremental changes we make; rarely do we engage in wholesale makeovers, but full of subtle improvements and recalibrations".

[2] An editor for Capital FM compared "Mirror" favourably to American singer Lizzo and that it is "the self-love bop we all need ahead of 'hot girl summer'".

[19][20][21] Femke Huurdeman, working for Spanish production company Canada, directed the music video, which premiered alongside the single on 26 May 2021.

[1][2] Throughout the visual, Sigrid performs various activities, such as spending time with a chameleon, shooting a bow and arrow and "channeling a living Rococo painting as she poses with a pomegranate" while being accompanied by a double.