[4] It was founded in 2015 by Jon Lansman, Adam Klug, Emma Rees and James Schneider after Jeremy Corbyn's successful campaign to become Labour Party leader and it was reported to have between 20,000 and 30,000 members in 2021.

According to Companies House, the current officers of Momentum Campaign (Services) Ltd are[9] solicitor Anthony Kearns (since 2019)[10] and co-chairs Andrew Scattergood and Gaya Sriskanthan (since 2021).

[3] Momentum maintains close ties with the Centre-Left Grassroots Alliance, a coalition of left-wing groups in the Labour Party who nominate joint candidates for the National Executive Committee and other internal elected positions.

[53] The group's present composition is:[54] Momentum was founded in 2015 by Jon Lansman and fellow national organisers Adam Klug, Emma Rees and James Schneider, four weeks after Jeremy Corbyn's successful campaign for the Labour Party leadership.

[61][1] Senior figures from Sanders' campaign included Erika Uyterhoeven (former national director for outer-state organising),[92] Grayson Lookner, Jeremy Parkin and Kim McMurray.

[105][failed verification] Momentum national treasurer Puru Miah, a councilor in Tower Hamlets, said that "we have put in place comprehensive systems so we can fully adhere to regulations".

[106] An April 2020 Momentum report analysed their performance in the 2017 and 2019 general elections, saying that constituencies such as Broxtowe, Hendon, Morley and Outwood, Southampton Itchen, Pudsey, Thurrock and Harrow East could have been won with the enhanced ground support they had by 2019.

[115] Momentum wanted the Labour manifesto to propose a four-day work week and a Green New Deal: mass state investment to address climate change.

[7] Momentum established a candidate network providing support for new councillors and encouraging women to stand, giving them opportunities to discuss policy and training in successful Labour-controlled councils such as Salford and Preston.

[95] Momentum additionally encourages Labour party members abroad in Europe, The United States and Australia to register British expats to vote and to participate in phone banking from where they live.

[133] The group developed their campaign strategy with three principles in mind, according to Emma Rees: people power, the targeting of marginal seats, and the idea of getting everybody to step up.

[136] Following their defeat in the 2019 general election, in which Labour lost 60 seats, Jeremy Corbyn and Momentum national coordinator Laura Parker resigned.

[143] Keir Starmer won the election in the first round (with Long-Bailey receiving 28 per cent of the vote), and Momentum failed to win any of the National Executive Committee by-elections.

[3] Critics of Corbyn in the Parliamentary Labour Party have raised concerns that Momentum might encourage the deselection of MPs and councillors who disagree with the organisation, in a process known as reselection.

[163] In July 2018, Momentum national coordinator Laura Parker said that four Labour MPs who voted against an opposition amendment to the EU withdrawal bill should be deselected.

Momentum has advocated greater membership involvement in party decision-making and "bolder reforms",[163][167] such as Labour members having a larger role in policy development.

[46] Momentum has pushed for lowering of the nomination threshold for Labour leadership candidates, so a left-wing successor to Corbyn could more easily contest a future election.

Under the Momentum plan, all Black or other-minority party members would automatically become part of BAME Labour and have a one-member, one-vote right to elect their NEC representative.

Momentum launched M.app, a smartphone app to alert delegates to key votes on the conference floor and send real-time information about events and rallies.

[170] Although a compromise motion lowering the threshold for local party and union branches to express dissatisfaction with an MP's performance from 50 to 33 per cent passed,[171] it was announced that the democracy review was being shelved.

[167] In the 2016 elections for the National Executive Committee, it emerged that Momentum, the Campaign for Labour Party Democracy and other leftist groups had formed a joint slate under the Centre-Left Grassroots Alliance (CLGA),[172] and were backing six representatives for the NEC: Rhea Wolfson, former chair of the Oxford University Jewish Society who replaced Ken Livingstone;[173] Christine Shawcroft, a senior Momentum figure who had been an NEC member for fifteen years and was briefly suspended from the party after defending Lutfur Rahman[174][175] (gaining media attention for jocular comments about dialogue with ISIS instead of air strikes);[176] Peter Willsman; Claudia Webbe; Ann Black, and Darren Williams.

[186] Four hundred and fifty CLPs participated, the highest turnout in Labour history; forty-two per cent of the selection nominations went to the Grassroots Voice slate.

[205] In the 2018 National Executive Committee elections, Momentum withdrew their support for Peter Willsman after an NEC meeting in which he described those "lecturing" the party about antisemitism as "Trump fanatics" who were "making up duff information without any evidence at all".

[206][207] In early 2020, two Momentum-backed Labour councillors in Brighton were suspended pending an investigation after accusations of antisemitism; the Green Party then led the minority administration.

[216] In 2021 and 2022 Momentum organised politically against the Covert Human Intelligence Act and the Police, Crime, Sentencing and Courts Bill (PCSC) by building connections between socialist MPs and social movements and supporting demonstrations and lobbying.

[217] In March 2021, then Momentum co-chairs Andrew and Gaya signed a Greenpeace open letter alongside other social movements opposing the PCSC Bill.

In August 2020, the organisation criticised Labour leader Keir Starmer's lack of support for those crossing the English Channel to claim asylum in the UK.

Corbyn had argued that Cameron's government lacked a credible plan for defeating Daesh, and the bombing in Syria would not increase the United Kingdom's national security; he has also said that military action should always be a last resort.

[221] In 2016, Momentum began The World Transformed (TWT): an annual four-day festival at the same time as the Labour Party conference which features art, music, culture and political discussions.

The festival has featured several international politicians present in different talks, such as: 2017 French presidential candidate Jean-Luc Mélenchon, leader of the German Die Linke party Katja Kipping and leader of the Tanzanian Alliance for Change and Transparency party Zitto Kabwe (in 2018),[224][225] US Democratic Congresswomen Ilhan Omar (in 2020), and US Independent Senator Bernie Sanders (in 2021) After the 2018 World Transformed event, local one- and multi-day festivals not organised by Momentum or TWT began to emerge.