It incorporates a piano sample from Pierre Bachelet and Hervé Roy's music from the softcore pornography film Emmanuelle, for which they were credited as co-writers, as the lyrics tackle the singer dealing with the departure of her boyfriend, while reminiscing about their time together.

The song was created in a New York studio, after Allen had met Ronson, who was impressed by her previous work and offered her different samples.

After obtaining commercial success with first two singles, "Smile" and "LDN", Allen decided to promote the album by releasing "Littlest Things".

The song was composed in New York, where Allen travelled after she had met producer Mark Ronson when at lunch with her then boyfriend, Seb.

[3] Regarding the process of composing the song, Ronson stated: We spent a day together and worked on a couple of things, and went to record shops where I played samples to her.

[3] Alex Petridis from The Guardian describes them as dealing with "misery and sex in equal measure", while reporting that Ronson asked Allen to sing in her London accent.

Heather Phares from AllMusic complimented the song, saying the ballad softens Allen's rough image and celebrates "the mundane moments of a dying relationship",[8] whereas Blender reporter Jon Dolan considers it a "forlorn single", where the singer writes about the terror of being in a relationship "with white-knuckled worry over a snip of florid piano from a '70s soft-core porn flick".

[9] While analysing Alright, Still, The Guardian reviewer, Sophie Heawood, described the lyrics as the "perfect description of missing someone: 'We'd spend the whole weekend lying in our own dirt / I was just so happy in your boxers and your T-shirt'.

"[10] Mark Pytlik from Pitchfork gave a positive review, saying that "'Littlest Things' is a supple piano-tickler that provides one of Allen's sweetest lyrical moments while simultaneously leaving Ms. Dynamite in the dust on the R&B balladeering front".

[1] IGN reviewer Todd Gilchrist named the song as the album's "most affecting tune", while thought it "compiles a sad and beautiful checklist of the intimate moments she no longer shares with her dearly departed boyfriend".

[12][13] It was Allen's lowest position on the UK Singles Chart at that time, being the only song to miss the top twenty, until "Who'd Have Known" peaked at thirty-nine in 2009.

The image zooms into the camera from the studio and turns the video black-and-white, with the story coming to life, as the singer begins the first verse.