The party was boosted in 1972 when three Members of the Stormont Parliament joined the party (one actually a nationalist). An Ulster Unionist/Conservative member of the Westminster Parliament also joined, providing Alliance with its only Westminster representation to date. Its first electoral challenge was the District Council elections of May, 1973 when they managed to win a respectable 13.6% of the votes cast. In the elections to Stormont which followed the next month the party polled 9.2% and won eight seats. The then party leader, Oliver Napier and his deputy Bob Cooper became part of the short-lived power sharing executive body.
The APNI's acceptance of the 1985 Anglo-Irish Agreement annoyed many of its supporters in the Protestant community and consequently they suffered at the polls. In 1988 the APNI's new leader, John Alderdice called for a devolved power-sharing government.
The APNI polled fairly poorly for the 1996 elections for the Northern Ireland Forum, and the 1998 election for the Northern Ireland Assembly winning around 6.5% of the vote each time. This did enable the party to win six seats in the Assembly.
John Alderdice resigned as party leader in 1998 to take up the post of the Assembly's Presiding Officer. He was replaced by Séan Neeson, who himself resigned as party leader in September, 2001. Neeson was replaced by current party leader, David Ford, a member of the assembly for South Antrim.
It was predicted that the APNI have suffered electorally as a new centrist challenger established itself in Northern Irish politics, the Northern Ireland Women's Coalition, whilst the main Unionist and Nationalist parties both moderated their position on cross-community co-operation, but in the Northern Ireland Assembly Elections, 2003, the APNI held all their seats, while the Womens Coalition lost both theirs. However the APNI's vote was heavily down. Several former prominent APNI members are now openly questioning whether the party now has a future in the province.
The APNI are linked with the British Liberal Democrats.