The album debuted at number one on the UK Albums Chart, breaking the world record for the youngest ever band with a number-one album, a record previously held by The Beatles. It reached the top of the charts after selling over 61,000 copies in its first week. The album has been certified 2× Platinum in the UK for sales of over 600,000 copies. The album won Best Album at the 2004 Smash Hits Awards. As of 2014, the album has sold over 2 million copies worldwide.
The album's main songwriters include band members Tom Fletcher and Danny Jones, as well as contributions from James Bourne from the band Busted, as well as some of Busted's main producers. The album is directly influenced by three things: modern pop punk, 1960s surfer pop, and tales of unrequited love. The track "Get Over You" is hidden, and does not appear on the track listing. It can be accessed by rewinding into the pre-gap, prior to the first track, "Five Colours in Her Hair". The track cannot be accessed if the album is played in a computer. Four singles were released from the album: "Five Colours in Her Hair" and "Obviously", which both went to number one, "That Girl", which reached number three, and the title track, "Room on the Third Floor", which peaked at five. A demo version of the track "Saturday Night" was previously released as "Saturday Nite" on the B-side of the "Five Colours in Her Hair" single. The international version of the album removes the songs "Broccoli" and "Surfer Babe" for contractual reasons. This version also features alternate artwork - the colour of the band logo is altered from yellow to red.
Sharon Mawer of AllMusic said of the album: "When a vacancy appears in pop music, it doesn't take long to fill it. As Busted moved out of pure pop into more serious material, the way was left open for McFly to capture teenage girls' hearts with their debut album, Room on the 3rd Floor. Named after the character in the Back to the Future trilogy and sounding like an updated 2000s garage band with close harmonies and raw guitars, McFly opened their album with two number one singles, "5 Colours in Her Hair" and "Obviously," so Room on the 3rd Floor really couldn't fail—and it didn't, hitting the top on its first week of release. "That Girl," the third single released, could have sat easily on a Beach Boys album from the 1960s, or possibly even the early Beatles with the count of "one two three four" before a guitar crashes in. "Surfer Babe" (fairly predictably) and "Down by the Lake" were also Beach Boys-sounding tracks. It's not that good, however—just a fun summery mid-2000s pop album from four lads who sound as if they were enjoying themselves singing mostly about girls. "Met This Girl" also took its influences from the Rolling Stones of the 1960s with a touch of Manfred Mann's fast-paced blues. "Everybody likes to party on a Saturday night," claim the lads on the track "Saturday Night," and if you were about thirteen years old and female, this was the album of summer 2004."
After a minor supporting slot with Busted, the band announced their very first headline tour in 2004. They started the tour just sixteen days after the release of the album's third single, "That Girl". The tour began on 22 September 2004, and ended on 13 October, with a total of fifteen sell-out dates.