[2] McSweeney built a reputation as a Labour organiser, leading successful campaigns to win a majority on the Lambeth London Borough Council and to defeat the far-right British National Party in Barking and Dagenham.
[10] From 2008 until 2010,[9] he campaigned with David Evans, Jon Cruddas, Margaret Hodge and Hope not Hate against the British National Party in Barking and Dagenham.
He worked with the council's leadership, developing communication strategies for the 17 wards in the community including the re-establishment of a duty on residents to keep their front gardens clean,[11] also focusing on patriotism and crime as campaign points.
[8] McSweeney became director of the think tank group Labour Together in 2017, reporting to a board that included Reed, Lisa Nandy, Jon Cruddas and Trevor Chinn, and also serving as company secretary.
[12] He composed a three-year plan for Starmer to become Prime Minister after taking control of the party, which involved first performing "immediate CPR" to reform the party's ranks (which included removing supporters of Corbyn and Scottish Labour leader Richard Leonard and excluding them from future leadership contests), then secondly becoming an effective opposition in parliament by directly attacking the Conservatives on their failures, and lastly winning power by outwitting the Conservatives on crime, defence and the economy.
[9] During this time McSweeney set up the Center for Countering Digital Hate, initially designed to target online antisemitism.
[13] Starmer succeeded in the 2020 Labour Party leadership election on 4 April 2020 with 56.2% of the vote and choose McSweeney as his chief of staff.
[19][20] Following Labour's victory in the 2024 general election on 4 July, McSweeney was appointed the head of political strategy alongside Paul Ovenden.
[23] Later that month, it was reported that Gray had moved McSweeney's desk further from Starmer's office twice and that she had requested that he be denied access to a secure computer system.
[25] In September 2023, New Statesman ranked McSweeney third on a list of the most influential left-wing figures in the UK and described him as Starmer's "most trusted aide".
[15] In the next edition of the list in June 2024, the magazine upgraded McSweeney to first place, naming him "the most influential person on the left today".
[26] In October 2023, The Times stated that "nobody without elected office wields as much power in British politics as McSweeney",[7] and The Guardian described him as "the most influential backroom operator in the party".