The band was formed at the 1999 Reading Festival by Okereke and Lissack. They went through a variety of names before settling on Bloc Party in 2003. Moakes joined the band after answering an advert in NME magazine, while Tong was picked via an audition. Bloc Party got their break by giving BBC Radio 1 DJ Steve Lamacq and Franz Ferdinand's lead singer, Alex Kapranos, a copy of their demo "She's Hearing Voices".

In February 2005, the band released their debut album Silent Alarm. It was critically acclaimed and was named 'Indie Album of the Year' at the 2006 PLUG Awards, which honours indie music. That year, the record was also certified platinum in the UK. The band built on this success in 2007 with the release of their second studio album, A Weekend in the City, which reached a peak of number two in the UK Albums Chart and number twelve in the Billboard 200. In August 2008, Bloc Party released their third studio record, Intimacy. The band are currently on hiatus as of 31 October 2009.

Russell Lissack and Kele Okereke first met in 1998 in Essex. Lissack had attended Bancroft's School, while Okereke attended Ilford County High School, then Trinity Catholic High School, Woodford Green for sixth form. They bumped into each other again in 1999 at Reading Festival and decided to form a band. Bassist Gordon Moakes joined after answering an advert in NME, and drummer Matt Tong joined after an audition. After going through a variety of names, such as Union, Superheroes of BMX, The Angel Range, and Diet, the band settled on Bloc Party in September 2003, a play on block party. The band has said that the name was not intended to be an allusion to the Soviet Bloc or the Canadian political party Bloc Québécois. However, Moakes said on the group's official Internet forum that it was more a merging of the eastern "Blocs" and the western "parties", in the political sense. He also notes that the name was not explicitly driven by politics, but rather it "looked, sounded, seemed fine so we went with it."

In November 2003, Bloc Party had their track "The Marshals Are Dead" featured on a compilation CD called The New Cross released by Angular Recording Corporation. They then released their debut single "She's Hearing Voices" on the then fledgling record label Trash Aesthetics. The band got their break after Okereke went to a Franz Ferdinand concert in 2003, and gave a copy of "She's Hearing Voices" to both lead singer Alex Kapranos and BBC Radio 1 DJ Steve Lamacq. Lamacq subsequently played the song on his radio show, labelling the track "genius", and invited them to record a live session for the show. The buzz generated off the back of the single led to another release, "Banquet/Staying Fat", this time through Moshi Moshi Records, and to the eventual signing with independent label Wichita Recordings in April 2004.

Bloc Party's debut album, Silent Alarm, was released in February 2005 and was met with universal critical acclaim. It was voted 'Album of the Year' for 2005 by NME, and reached number 3 on the UK Albums Chart before being certified platinum. The first single from the album, "So Here We Are/Positive Tension", made the top 5 on the UK Top 40 chart. Further singles "Banquet" (which reached number 13 in NME's 'Top 50 Singles of 2005'), "Helicopter", and "Pioneers", whilst failing to repeat this success, still managed to reach the UK top 20. The animated video for "Pioneers", made by the Shoreditch-based Minivegas design agency, was top of the NME video charts for 4 weeks.

The band received positive reviews from critics in the United States and they toured there heavily in the 18 months that followed the release of Silent Alarm. In early 2006, they finished their tour with sold out shows in Los Angeles, Miami and Berkeley. The album went on to sell more than 350,000 copies in North America and over a million worldwide. After this success, the established electronic group, The Chemical Brothers, soon collaborated with Okereke for "Believe", a track on their Push the Button album. An album of remixes of tracks from Silent Alarm had also been released at the end of August 2005 in the UK. This remix album, entitled Silent Alarm Remixed, retained the album's original track list and includes remixes from the likes of Ladytron, M83, Death from Above 1979, Four Tet, and Mogwai.

During July 2005, Bloc Party recorded two new tracks with Silent Alarm producer Paul Epworth. The songs were released as single with a B-side, titled "Two More Years", to coincide with the band's October 2005 UK tour. The tour was also accompanied by a re-issue of Silent Alarm, which included "Two More Years" and former single "Little Thoughts" as bonus tracks. A remix of "Banquet" by The Streets, as well as a music video for the song, were included in the "Two More Years" single. Bloc Party also contributed the track "The Present" to the Help!: A Day in the Life compilation, the profits of which benefited the War Child charity.

Bloc Party's second album, A Weekend in the City, was produced by Garret "Jacknife" Lee. It was released in February 2007, although it was leaked in November 2006. It became available for download on the UK iTunes store before the physical release, and reached the number 2 spot on the UK Albums Chart. The album also reached number 2 on the Australian and Belgian charts, and debuted at number 12 in the Billboard 200, with 48,000 copies sold. The first single, "The Prayer", was released on 29 January, and became the band's highest charting single in the UK Top 40, reaching number 4. In the build up to the release of the album, BBC Radio 1 DJ Zane Lowe aired a live set by the band from Maida Vale studios on 30 January 2007, featuring a mix of old and new songs. On 1 February 2007, A Weekend in the City was made available to listen to for free through the Bloc Party's official MySpace page.

The next single, "I Still Remember", was Bloc Party's highest charting American single, peaking at number 24 on the Modern Rock Chart. The band released their third single, "Hunting for Witches", with an accompanying video clip in August 2007. The single became their only ARIA Chart entry, peaking at number 20. In October 2007, it was announced that Bloc Party would release a new single, "Flux", on 13 November—ahead of their end of year gigs. The electronic song, also produced by Jacknife Lee, was very different from previous singles released by the band.

The band's first gig following the release of A Weekend In The City was on 5 February 2007, in Reading, and was broadcast live on BBC 6 Music. On 20 May 2007, Bloc Party headlined on the In New Music We Trust stage at the BBC Radio 1 Big Weekend in Preston. They also performed at the UK leg of Live Earth on 7 July 2007 at Wembley Stadium. Furthermore, the band played sets at T in the Park and Oxegen 07 that same weekend, as well as Glastonbury and the Reading and Leeds Festivals later in 2007. Bloc Party announced a tour of Australia and New Zealand in August 2007, which would include a special appearance at the Splendour in the Grass Festival on 5 August. On 17 September 2007, they recorded a set for the PBS show Austin City Limits. a day after playing at the Austin City Limits Music Festival. On 27 October, the band performed a set at London's The Roundhouse with the Exmoor Singers, a London based choir, as part of the BBC Electric Proms. The set included songs from both Silent Alarm and A Weekend In The City along with the first UK live performance of "Flux".