He is best known for having worked on Jeremy Corbyn's successful 2015 campaign for the leadership of the Labour Party and subsequently founded the pro-Corbyn organisation Momentum.

Lansman ran for Union President on the same slate as Andrew Marr, then nicknamed Red Andy, who provided campaign cartoons.

[12] During the deputy leadership election campaign, on the Weekend World discussion programme of 21 September, Denis Healey wrongly accused Lansman of organising the severe heckling of speeches he had given in Cardiff and Birmingham.

[16] Mervyn Jones identified Lansman as part of a group on the left that were "quite prepared to see a right-wing breakaway as the necessary cost of swinging the party in what they saw as the desired direction".

[17] Alongside Vladimir Derer and Victor Schonfield, Lansman was described as "unreservedly dedicated", with "no political ambitions of [his] own" and "in a position to work day and night for the cause without pay".

[17] Lansman later organised Benn's campaign in the 1988 Labour leadership election, unsuccessfully challenging incumbent Neil Kinnock.

[21] During the election, he was criticised for posting a link on Twitter to a Facebook page depicting rival candidate Liz Kendall as a future Conservative Party leader.

"[26] A few days later, in early May, he wrote on the Left Futures blog that the use of the term Zionist to describe supporters of the government in Israel was "counter-productive".

He cited one poll of British Jews which found 71% favoured a Palestinian state and 75% opposed the Israeli settlements, while 68% still identified as Zionists.

Jeremy Corbyn said the Labour Party settlement with the staff who appeared in the Panorama programme was a "political decision, not a legal one".

[33] However, Lansman withdrew, saying that he had stood to avoid Jennie Formby being chosen unopposed and to help end Labour's "command and control" inclinations,[34] and that he had achieved his intention of creating a debate over how the party is managed.

He also criticised leaders of the PSC for supporting a one-state solution in Israel–Palestine and for its inadequate response to Hamas's attack on Israel on 7 October 2023, as well as criticising elements of the British left for dividing Jews into a binary distinction of Zionists or anti-Zionists, saying "very many who the left might see as 'Zionists' are critical of illegal settlements, the occupation, of the Israeli government and even of the conduct of the present war".

[42] Lansman has been a trustee of the charity, Breast Cancer Care, under whose auspices his wife had set up the Lavender Trust before she died, both to help younger women become aware of the disease and to support them.