Jack's Mannequin is the solo project of Andrew McMahon who is also the vocalist, pianist and main songwriter of the band Something Corporate. The name Jack's Mannequin originated from one of McMahon's songs called Dear Jack, which was written about a friend of his whose brother was diagnosed with childhood leukemia. Andrew himself was diagnosed with leukemia on June 1, 2005 and cancelled a show for the first time ever when he received the call from his doctor.

The initial impulse behind McMahon's solo project Jack's Mannequin was Locked Doors, a song he recorded in December 2003, which he felt was too different from previous Something Corporate material to be one of their songs. He realized that, if he would ever decide to release it, it would be on a solo album rather than a Something Corporate record. The idea seemed to be off the table, until in late summer 2004 both McMahon and his bandmates were exhausted from months of touring. They decided to take a temporary break. During his time off, McMahon started writing songs, knowing they would not see the light of day as Something Corporate songs. These songs were a more therapeutic approach McMahon took, and therefore a more personal and intimate testament of his songwriting.

Over the next couple of months, McMahon began recording the newly written songs with producer Jim Wirt, who would play guitar and bass over sampled drum tracks, while McMahon contributed vocals and piano. They later brought in additional musicians such as Bobby anderson (formerly of River City High) and Tommy Lee to add to the mix. In March 2005, Jack's Mannequin (now consisting of McMahon, Jon Sullivan on bass, Anderson and jacques brautbar on guitar and Jay McMillan on drums) played their first live show at Molly Malone's Irish pub in Los Angeles, California.

Their debut album Everything in Transit was released on August 23, 2005 debuting at #37 of the Billboard 200 with over 22,000 copies scanned within its first week.

Their first album, titled Everything in Transit, was released in August 2005 and received a positive response from fans and critics alike. A summer tour was scheduled to support the record release, but was cancelled after McMahon was diagnosed with acute lymphoblastic leukemia and had a bone marrow transplant. The band began recording their second album in the summer of 2007. In August 2007, McMahon posted the lyrics to the song "Cellular Phone" on his blog,which the band played live at several shows in 2007 and 2008. Also, in February 2008, videos of live performances of two songs off the new record ("Caves" and "Suicide Blonde") were uploaded onto YouTube for public viewing. McMahon appears on the cover of Alternative Press's January 2008 issue, which contains an interview with him.The new record, revealed to be titled The Glass Passenger in an official blog post,had an original release date of April 22. The date was then tentatively set to June 3 and is now expected to be on shelves by the end of August or beginning of September, 2008. McMahon told the Montclair State University student newspaper that the new album just isn't finished yet, and won't be until the end of May - he then needs two to three months to get the word out to the press and media.

On May 5, 2008, Paramore singer Hayley Williams announced on The Sauce that Paramore and Jack's Mannequin had a tour scheduled together, to begin at the end of the summer. Dates have been confirmed for August and September 2008.