They originated from those former supporters of the Revolutionary Communist party who continued to look to Ted Grant for leadership. After he was expelled from Gerry Healy's group The Club in 1950 they reorganised as the Revolutionary Socialist League in 1953 and affiliated to the International Secretariat of the Fourth International in 1957.
Less intent on a policy of entrism into the Labour Party than the other Trotskyist groups initially they became convinced of the need for this tactic during the 1950s. this was signaled by their document Problems of Entrism published in 1959.
The newspaper The Militant was founded in 1964, when the majority of the group broke from the International Secretariat of the Fourth International with the minority forming the International Group which was to develop into the International Marxist Group.
Militant had a period of steady growth and splits throughout the late 1960s and the 1970s. In the 1980s they started taking control of various Labour Party organisations (they had sporadic control over the Labour Party's youth movement, the Young Socialists) and Constituency Labour Parties. In the mid 1980s a Militant faction took over the ruling Labour group on Liverpool city council. The strong links between the Militant base of Liverpool and Northern Ireland also ensured that the Labour Party of Northern Ireland (which was not linked to the mainland Labour Party) was strongly influenced by Militant.
Although they did not really share the multicultural analysis of much of the rest of the Labour Party left, they were a visible component of that coalition. Many Labour figures saw the Militant tendency as a primary reason for their "loony left" image, as portrayed by the right-wing press. Ineffective attempts to control Militant were made by party leader Michael Foot in the early 1980s, and were carried on with more vigour by Neil Kinnock. In what many people saw as a crucial stage in the turnaround of Labour, Kinnock made a speech to the Labour Party Conference in 1985 that attacked Militant entrism and their record in Liverpool Council.
The Militant tendency were very active, like many other left wing groups, in the campaign against the Community Charge or "Poll Tax".
Over the next couple of years the Labour Party machinery successfully purged Militant so that the majority of the Militant group abandoned entrist tactics and formed themselves into a separate party, first calling themselves Militant Labour (and in Scotland, Scottish Militant Labour) and latterly in England the Socialist Party, a constituent element of the Socialist Alliance which they later left in 2002. The minority in Militant, led by its founder Ted Grant and by Alan Woods went on to set up the Socialist Appeal group and the In Defence of Marxism website. The Scottish Socialist Party is led by former Militant activist Tommy Sheridan and has six MSPs.