Lansman founded Momentum, a grassroots left-wing organization supporting Jeremy Corbyn's leadership of the Labour Party, in 2015, and chaired it until 2020.

A longtime Labour activist, he helped organize Tony Benn's 1981 campaign for deputy leader of the party.

• He was elected to Labour's National Executive Committee in 2018, and edited the left-wing blog Left Futures.

• Raised in an Orthodox Jewish family in north London, he has at times criticized parts of the left over antisemitism.