[5] The music on the album has been compared to notable acts in the United Kingdom like the Sugababes in the early stages of their careers,[6] Basement Jaxx,[7] Kathy Diamond,[7] and Lily Allen.

[9] The lyrical content is centered on themes of relationships turning bitter and envious,[10] boy troubles,[11] and clubbing in London,[5] noted by many critics, from a rare genuine and frank perspective.

[14] "Power on Me" displays vulnerability in Katy's vocals[17] and is built over a funky house groove[5] with synth tones similar to those by Axel-F.[17] "Go Away" is structured as a downtempo[18] trip hop[15] and trance[10] ballad.

[20] Natalie Shaw of BBC Music wrote that it "moves through house, dubstep, drum’n’bass, rave, UK garage and RnB with ease, attacking each with such accessibility it’s not hard to imagine each track topping the singles charts".

She described the production as "sharp and cutting throughout, never losing its feverish hi-NRG" and noticed that Katy B is "a new breed of singer, adding a vibrant gloss to a new combination of sounds with a charm and personality all of her own".

[13] Nate Patrin of Pitchfork called her "a genre-spanning pop singer who isn't tied down to a single thing, no matter how well it suits her" and felt that "what puts On a Mission over the top is Katy's way of expressing herself with emotions that extend past "wooo, druuunk" into more nuanced and detailed relationships with booming systems and the people who flock to them".

[14] Ben Hogwood of musicOMH praised Brien's ability of "projecting emotion" as well as her voice, saying it "could easily sing the front page of the South London Press and have you clamouring for more" and called the lyrics "brilliantly written, at times extremely profound while at others throwing off the shackles for a night's partying".

[23] Jon O'Brien of AllMusic defined it as "a strikingly self-assured and original affair which not only convincingly captures the sound of contemporary London nightlife, but also reflects the voices of much of her late-teens/early-twenties generation".

[15] Noel Gardner from Drowned in Sound called it "a sterling debut" and described it as "a pop album which comes off as written from life while also addressing the concerns of its audience".

[12] Kitty Empire from The Observer called the songs on the album "12 solidly built tunes that reflect her love of the euphoric rush of clubbing, of falling in love and the comedowns in their wake",[16] while Simon Price of The Independent described On a Mission as "a debut album of subtle, mature, intelligent electronic pop, reminiscent of Sugababes before it all went wrong" and felt that it "sounds like a modern pop classic".

[7] Rick Pearson of the Evening Standard called it "thrilling, original and utterly of the moment" and stated that "dance music has discovered a new star".

[5] Gavin Martin of the Daily Mirror wrote that Brien "effortlessly glides through a template that mixes dubstep, garage, house and grime on this appealing, user-friendly debut",[8] while Robert Copsey from Digital Spy called the album "a collection of well-groomed songs loaded with earworm hooks, rich melodies and on-point lyrics that focus on music itself rather than homing in on a particular trend".

[9] Thomas H. Green of The Daily Telegraph described it as "efficient, likeable, club-friendly pop, with the house numbers less memorable than the drum and bass leanings".

"Lights On", featuring Ms Dynamite, was released as the album's second single on 10 December 2010 through Rinse FM and Columbia Records, where it debuted in the UK at number 4.