After partition the NILP was founded as a socialist political party by groups including the Belfast Labour Party and found its main bed of support amongst working class voters in Belfast. It initially declined to take a position on the "Border Question" and instead sought to offer itself as an alternative to both nationalism and unionism. It maintained relations with the British Labour Party who did not allow membership or organisation in Northern Ireland until 2004. In the 1925 Northern Ireland general election, the party secured three seats in Belfast, including William McMullen elected in Belfast West, as well as Sam Kyle and Jack Beattie ; this was the last election for the Northern Ireland Parliament using the single transferable vote system. The party had a Westminster Member of Parliament on only one occasion, when Jack Beattie won the 1943 Belfast West by-election, retained the seat in 1945, but lost it in 1950. He regained the seat as an Irish Labour Party candidate in 1951. In 1949, following the declaration of a Republic in the south, the Northern Ireland Labour Party's conference voted in favour of the Union with Great Britain. The result was a sharp decline in the party's already limited electoral success, as Catholic voters deserted, and the Irish Labour Party attempted to organise in Northern Ireland. An earlier refusal to adopt this policy had split the party, with leader Harry Midgley forming his own strongly Unionist Commonwealth Labour Party. Later in the 1950s, the party began to gain ground amongst unionist voters, and after the breakup of the Irish Labour Party's new attempts to organise in Northern Ireland among some nationalists, it saw its greatest period of success between 1958 and 1965. Four NILP MPs were elected to Stormont in 1958 for Belfast constituencies: Tom Boyd, Billy Boyd, Vivian Simpson, and David Bleakley. The NILP then became the official opposition at Stormont. Success came despite continued divisions, over such matters as Sunday Observance – two NILP Belfast councillors voted to close the city's park playgrounds on Sundays and were expelled as a result.
In 1971 the new Prime Minister of Northern IrelandBrian Faulkner appointed NILP former Stormont MP David Bleakley to his Cabinet as Minister of Community Relations, in an attempt to bring reforms to Northern Ireland. However, the following year the Stormont Parliament was suspended when it resisted the London government request to take over responsibility for public order. In the 1973 referendum on the border, the NILP campaigned for Northern Ireland to remain in the United Kingdom. David Bleakley was elected to the 1973 Assembly and 1975 Forum for East Belfast. The Northern Ireland Labour Party continued to contest elections but with a dwindling support base. It broadly supported the Ulster Workers' Council strike in 1974, and shortly afterwards adopted a policy of unionism. Alan Carr became its leading figure from the mid-1970s until the early 1980s, by which point it had only about 200 members, and just a single councillor was elected for the party in 1981. A party conference in 1983 narrowly failed to secure a necessary two-thirds majority to wind up the party, but it stood no candidates in the 1983 general election, its Chairman and Party Secretary having resigned just beforehand, and by the 1985 Northern Ireland local elections, its three candidates received no support from the central body.
