Cairns had ambitions to enter House of Commons but was barred due to the House of Commons Act 1801 and the Catholic Relief Act 1829 which prevented present or former Roman Catholic priests from being elected to Parliament. To rectify this, Siobhain McDonagh MP introduced the House of Commons Disqualification Bill in Parliament on 16 June 1999, but the Bill failed. The government subsequently introduced the House of Commons Bill, which removed almost all restrictions on clergy of whatever denomination from sitting in the House of Commons. The only exception is Church of England bishops, due to their reserved status as members of the House of Lords. The bill passed on 11 May 2001. Cairns had already been selected as the Labour candidate in his home town following the retirement of Norman Godman. He was elected as the Labour MP for Greenock and Inverclyde at the 2001 General Election with a majority of 9,890, becoming the first person born in Greenock to represent it in Parliament. He made his maiden speech on 4 July 2001. Cairns was appointed the Parliamentary Private Secretary to the Minister of State at the Department for Work and PensionsMalcolm Wicks in 2003, and following the 2005 General Election, at which, due to the redrawing of boundaries his constituency was abolished and replaced with a larger Inverclyde constituency, he became a member of the Labour government as the Parliamentary Under-Secretary of State for Scotland. He then had the Northern Ireland Office added to his responsibilities and in 2007 he became the Minister of State at the Scotland Office. Cairns was Chair of Labour Friends of Israel, and while he gave up the position when becoming a junior minister, he remained a committed member of the group. On 16 September 2008, Cairns resigned from the government during arguments in the Labour party over Gordon Brown's leadership, saying that the time had come to "allow a leadership debate to run its course". The Inverclyde MP was the first minister to resign after rebel MPs began calling for a leadership contest. In the 2010 General Election, Cairns was returned as Member of Parliament for his constituency of Inverclyde with a majority of 14,416, which was an increase on his previous election.
Cairns was openlygay. He was taken to hospital in March 2011, suffering from acute pancreatitis, and died on 9 May. He is survived by his partner, Dermot Kehoe.