

Billboard noted the mixtape is more introspective than RM's first mixtape, labeling "Moonchild" a "smooth bilingual alt R&B track", "Badbye" as "sorrowful", and "Uhgood" "pensive". Vulture noted the mixtape contains "the slowest of bangers" and "moody bops", with RM whisper-rapping on "Forever Rain".

Vulture stated "these tracks are the free therapy your body, soul, and bank account collectively require to ease into whatever fresh hell this week will inevitably bring", also saying Mono "sounds like how settling into a lukewarm bath feels: cleansing, just the right amount of steamy, and soothing to the touch". MTV called the mixtape "equal parts wistful and weary, and the theme of loneliness is constant throughout", and said RM lays "his deep insecurities bare on songs like 'Tokyo' and 'Seoul'". Critical reception Professional ratings Review scores Mono is RM's second mixtape, following the release of his 2015 self-titled RM, and the first of his mixtapes to be released commercially.

It includes a track with production by Honne, as well as features the South Korean indie rock band Nell and singer eAeon. The track listing was later shared by RM himself, with Mono described as a "playlist" rather than a traditional mixtape. The mixtape itself was first announced on October 20, 2018, along with its track listing, album cover, and release date, through the Big Hit Entertainment Twitter page. They initially gained an interest in collaborating with RM after discovering he was a fan of their music online, and meeting him in Seoul after performing there. In June 2018, British electropop duo Honne said in an interview that they had possibly worked on music with RM.

RM first teased the mixtape and confirmed its production in March 2018 by posting a clip of an unnamed song through BTS' Twitter account. It features songs sung and rapped in both Korean and English. RM has called Mono a playlist, while other publications have referred to it as a mixtape. It features seven tracks and was accompanied by an animated music video for the final track, "Forever Rain". It was released on October 23, 2018, through Big Hit Entertainment. Mono (stylized as mono.) is the second mixtape by South Korean rapper RM.
