It was founded in 1982 when the International Marxist Group entered the Labour Party. Officially named the Socialist League, it became universally known by the name of its publication, Socialist Action.
Its character changed in a wave of splits in the mid-1980s, beginning in 1985 when a minority, led by Phil Hearse, Dave Packer, Davy Jones, and Jane Kelly formed the International Group, which in 1987 merged with the Socialist Group to form the International Socialist Group and publish Socialist Outlook.
The remaining majority of the Socialist League consisted of two factions. One, pro Socialist Workers Party (USA) faction, was part of the Pathfinder tendency. The Pathfinder tendency was expelled by the Central Committee which was led by John Ross. Both sides contend that they would have been the conference majority if the explusions had not happened.
The remainder of the group drew pessimistic conclusions from the fall of the Stalinist regimes in the Soviet Union and Eastern Europe. It continues to define itself as a Trotskyist group. It considers the Soviet model to have been preferable to capitalism for the working class, but it has always criticised that model for its bureaucratic and undemocratic features, accepting Trotsky's definition of the USSR as a degenerated workers' state. At the time of the split, the group was given equal status within the United Secretariat of the Fourth International with the International Group. However, Socialist Action participated in the 1989 and 1990 FI Youth Summer Camps but suffered another split after the 1991 World Congress. Small groups of Socialist Action members regularly resigned and joined the Internationl Group, and its successor the International Socialist Group, between the original split in 1985 and the 1991. At the 1995 world congress the ISG replaced Socialist Action as the British section.
Working with increasing secrecy in the Labour Party, often under the auspices of other apparently independent organisations, its members became supporters of Ken Livingstone and the Socialist Campaign Group of Labour MPs.
In 2001 it stopped publishing its journal, also named Socialist Action, but continue to organise as a faction, for instance as the Student Broad Left. Some of its activists played leading roles in organising the 2004 European Social Forum. Additionally its members continue to publish occasional pamphlets and leaflets.
It is present on a small number of British university campuses, especially Goldsmiths College. Its attempt to win key posts in the London School of Economics Students Union has failed in recent years.
The group came under closer scrutiny in 2008 when its relationship with Ken Livingstone and operation within the Greater London Authority was documented by a number of newspapers and a TV documentary for Channel 4's Dispatches programme Livingstone referred to the claims in the Dispatches documentary as a hatchet job