21 received mildly positive reviews from critics, who saw it as an improvement over his debut album O (2005). The album debuted at number 1 on the US Billboard 200, selling 119,000 copies in its first week of sales, making it his second album to debut at number 1, although selling 60,000 less than his debut album. It has sold over 390,000 copies in the United States up to November 2008. The album spawned two singles: "Entourage" and "Ice Box".
21 received mildly positive reviews from music critics. Andy Kellman of AllMusic gave it an "Album Pick" title, praising the production throughout and Omarion for putting more input as a songwriter and performer, saying that he "truly surpassed his status as the former member of a boy band." DJ Z of DJBooth commented on the lyricism feeling mature and the production showing more sophistication, saying that "Omarion’s second album is a welcome addition to his musical catalog and should appeal to both his fan base and a more adult crowd." Thomas Inskeep of Stylus Magazine also praised the album for its production and having a good ratio of solid-to-great songs. Putting it alongside Ciara's The Evolution for "best straight-up R&B album" he said that "the pair leave plenty of encouragement about the future of the genre."
Damien Scott of Vibe gave a mixed review, saying that Omarion lacked range in his performance and that he's "never fully in charge." Mark Edward Nero of About.com was also mixed towards the album, praising the first two singles but found the other tracks above-average at best, in terms of production and vocal quality that "range from average ("Obsession," Made For TV,") to fair ("Midnight," "Beg For It")." He concluded with: "Although O's taken a step forward in establishing himself as an adult solo artist, he still hasn't completely arrived yet. But at only 21 years of age, he's still got plenty of room to grow."