The band is best known for their hit single "We Are Young" featuring Janelle Monáe, which reached number one on the Billboard Hot 100 and Digital Songs charts. It became the first rock/alternative song to top the Billboard Hot 100 chart since Coldplay's Viva La Vida in 2008.
In February 2008, The Format split, and Nate Ruess immediately asked Andrew Dost, of Anathallo, and Jack Antonoff, of Steel Train, to join his new project. Dost had previously toured with The Format, providing various instrumentation, and Ruess knew Antonoff after The Format toured with Steel Train. The three began working together in New Jersey within a week, with Ruess singing melodies for the other two to provide music for. The first song the band recorded a demo of was "Benson Hedges," which was then made available for free in Spin's September 20, 2008 article on the band. Fun approached Steven McDonald, who had previously produced The Format's Dog Problems with Ruess, to produce their debut album. McDonald, who also played bass guitar on the album, was enthusiastic about the project, at one point saying, "I can’t believe what we’re working on here. This crushes anything I’ve ever done."
Recording took place in the Fall of 2008, and the band's first single, "At Least I'm Not As Sad (As I Used To Be)" was made available as a free download on the band's Myspace page on April 6, 2009. Aim and Ignite was released August 25, 2009. Aim and Ignite was released to positive reviews. AbsolutePunk.net's Drew Beringer praised the album, calling it "what a pop album should sound like" and "the most essential pop album of 2009". Allmusic called the album "progressive, but in the best possible way" and admired Ruess's lyrics for "investigating the larger truths of life...with a witty approach that keeps the songs bubbling merrily along on a positive note". Dave de Sylvia of Sputnikmusic wrote, "Aim and Ignite isn’t the most consistent pop album around," but he ultimately commended the album as "a superbly mixed and arranged album made by musicians who clearly understand the limits and potential of pop music". Estella Hung of PopMatters was less impressed with the album, praising songs "Be Calm" and "The Gambler", but criticizing the lyrics and production of the album's early tracks. Hung concluded that while Aim and Ignite is "pretty original to say the least", it "fails to live up to the Format’s last outing". Popdose's Ken Shane called the album "an interesting and unusual listen". Shane applauded the album's songwriting and said "many of the songs are really good", but he objected to the "cute" production, desiring to hear the band "in a more stripped-down form. He ended his review with: I have a similar problem with Dr. Dog, a band that was recommended to me by a number of people. I think much of their recorded work is too fussed over, but when I saw them live and their sound was more stripped down out of necessity, emphasizing their powerful songwriting, I thought they were wonderful. Perhaps the same fate awaits me with Fun." The album reached number 26 on Sputnikmusic's top 50 albums of 2009. The album peaked at 71 on the US album charts.
Fun began its first North American tour on November 8, 2008 with Jack's Mannequin. The debut album Aim and Ignite was released on August 25, 2009. In reviewing the album, The Washington Post called some of the arrangements "theatrical, much like those on Panic! at the Disco's 2005 debut". In February 2010, Fun supported Jack's Mannequin on their headline tour, along with Vedera which was followed by their first UK appearances in March. Their current touring band also includes Rob Kroehler, Emily Moore, Nate Harold, and Jon "Jonny Thunder" Goldstein. In April 2010, Fun supported Paramore's headline tour. Other support for the tour includes Relient K and Harriet Ellis of Summer Beats. The band then embarked on a full UK tour in May. On August 4, 2010, Fun announced that they had signed with label Fueled by Ramen.
On August 14, 2010, it was announced via the Official Paramore Fan Club that Fun. would be opening for Paramore on their November UK tour. In 2010 Fun's single, "Walking the Dog," was used in a commercial for the travel site Expedia.com. In 2010, Will Noon (formerly of Straylight Run) played drums with Fun on tour, according to Noon's twitter page. To celebrate the Paramore UK tour and the band’s new single ‘Walking The Dog’, Hassle Records gave away a free download of an acoustic version of the track. On April 27, 2011, a video of the band performing a new song entitled "Carry On" was uploaded to YouTube.
On May 17, 2011, the band released "C'mon" as a joint single with Panic! at the Disco, who they opened for on their 2011 Vices & Virtues Tour.
On September 14, 2011, the band announced that the first single off of their next album would be called "We Are Young" and would feature Janelle Monáe.
On November 7, 2011, the band announced that their next album would be titled Some Nights and released on February 21, 2012.
On December 6, 2011, the band's song "We Are Young" was covered by the TV show Glee in the episode "Hold On to Sixteen", after the end of Sectionals in the auditorium of McKinley High. The single for the Glee version of the song hit #1 on iTunes that night. On December 12, the song was used to highlight the nominees for Game Changer of the Year at the 2011 edition of the WWE Slammy Awards.
On December 12, 2011, the band's song "One Foot" was available for instant streaming and free download on Nylon's website.
On December 30, 2011, the song "We Are Young" was played in the TV show Chuck in the episode Chuck versus The Baby, in the final few minutes during the wrap up of the episode's story lines.
On January 27, 2012, the song "We Are Young" was featured in the Chevrolet Chevy Sonic Super Bowl XLVI commercial "Stunt Anthem," also featuring American professional skateboarder Rob Dyrdek and the rock band OK Go.
On February 13th, the band released an album stream on their website along with a note from Ruess thanking fans for their ongoing support. Ruess states he is "over the moon about what you're about to hear and falling asleep knowing that as soon as I wake up, this will no longer be a dream."
On February 21st, Fun. released their album "Some Nights", along with their music video to the song "Some Nights Intro".
On March 7th, Fun.'s Single "We Are Young" reached the No. 1 spot in the Billboard Hot 100 Charts. This makes Fun. the first multi-member Rock Band to make a Billboard debut with the #1 spot in the Hot 100 since Nickelback's "How You Remind Me" in Dec. 2001/Jan. 2002. On the same day, "We Are Young" was used by Apple in a promotional video of their recent retail development in Grand Central Terminal.
On March 19th, Fun. was announced as a performing band at the Bonnaroo 2012 music festival.