It was released on February 16, 2022, through Sony Music Entertainment Japan, as the first single from the short story collection project Hajimete no.

Written by Ayase and based on 159th Naoki Prize-winning Rio Shimamoto's short story Watashi Dake no Shoyūsha, the song is about the desire of an android for their owner.

On December 1, 2021, the same day as the EP The Book 2 release, Yoasobi announced a collaboration with four Naoki Prize-winning novelists: Rio Shimamoto, Mizuki Tsujimura, Miyuki Miyabe, and Eto Mori; for writing and singing songs based on the novelists' short stories under the theme of "a story to read when you first did".

[3] On January 26, during their radio show Yoasobi's All Night Nippon X, the duo revealed the title of the song "Mr.", based on Shimamoto's story.

[10] "Mr." is described as a "painfully ephemeral" city pop song about the desire of an android for their owner, written by Ayase, a member of Yoasobi.

[19] Produced by Ijigen Tokyo, who previously handled Yoasobi's "Halzion" music video,[20] the music video depicts the android Boku reminiscing a memory with their owner Mr. Naruse before the owner is severely injured and dies in a large-scale catastrophe.