She was Member of Parliament for Halifax from 1987 until 2005. She is a left-winger who was a member of the Socialist Campaign Group and is a Eurosceptic. A frequent rebel against the Labour government elected in 1997, Mahon stepped down from the House of Commons at the 2005 general election and was succeeded by a Labour councillor, Linda Riordan. She resigned from the Labour Party in April 2009, saying she can no longer tolerate how the party operates. Mahon, who has a history of peace activism, opposed the Iraq War and missile defence plans in her time in office. She sought to protect benefits for parents, women's rights (particularly in regard to abortion), and gay rights. Mahon was also a supporter of reform of the House of Lords.
She attended a grammar school in Halifax. In 1979, she gained a BA in Social Policy from the University of Bradford. She worked in the NHS as a nursing auxiliary for ten years. She taught Trade Union Studies at Bradford College from 1980 to 1987 and was a councillor on Calderdale Council.
In November 2005, a film documentary by Sigfrido Ranucci of Italy's Rai News 24, The Hidden Massacre, asserted that the US military had used White Phosphorus (WP) as an incendiary weapon, including against civilians in Fallujah during operation Phantom Fury. The RAI documentary also quoted a 13 June 2005 UK MOD letter to former Labour MP Alice Mahon stating that:
"The US destroyed its remaining stock of Vietnam era napalm in 2001 but, according to the reports for 1 Marine Expeditionary Force (1 MEF) serving in Iraq in 2003, they used a total of 30 MK 77 weapons in Iraq between 31 March and 2 April 2003, against military targets away from civilian areas. The MK 77 firebomb does not have the same composition as napalm, although it has similar destructive characteristics. The Pentagon has also told us that owing to the limited accuracy of the MK 77, it is not generally used in urban terrain or in areas where civilians are congregated".
Mahon acted as a defence witness in the trial of Slobodan Milošević in 2006. Following the testimony of Slobodan Jarčević, who was foreign minister of the self-declared Republic of Serbian Krajina, RSK, in modern-day Croatia, from October 1992 until becoming foreign policy advisor to the RSK president Milan Martić in April 1994, Milošević called Mahon, who was a member of the British parliament throughout the 1990s and also sat on the NATO parliamentary committee from 1992 onwards.
In 1999, she said:
Having visited Yugoslavia, I feel as strongly about the innocent civilian victims of laser-guided bombs as I do about victims of ethnic cleansing and the Albanian refugees who must have the right to return home in safety
Mahon suffers from age-related macular degeneration (AMD), an eye disease which destroys the central part of the vision in the eye, making the sufferer ultimately blind. According to the RNIB, more than 18,000 people in Britain go blind every year due to the condition making the disease the leading cause of sight loss in Britain. Mahon lost most of the sight in one eye due to AMD, and expects to lose sight in the other. Calderdale Primary Care Trust has refused to fund a drug which could stabilise or improve Mahon's condition, and so she has taken to a high-profile publicity campaign and threatened to take the PCT to the High Court in 2007.
Mahon resigned her membership of the Labour party in April 2009 saying she can no longer condone how it operates.
In her letter to the Halifax Constituency Labour Party she criticised the Prime Minister: "This Labour Government should hang its head in shame for inflicting [the Welfare Reform Bill] on the British public just as we face the most severe recession any of us have experienced in a lifetime." The Bill has been criticised by a number of disability campaign groups and Labour MPs for not helping the disabled or unemployed. Mahon said that she "totally disapproved of everything Tony Blair was doing" and was dismayed at the impotency shown by the government in tackling energy providers and financial institutions. She condemned the failure of the party to stick to its election manifesto, including pledges not to privatise the Royal Mail, and to give the country a referendum on the EU Constitution (which later became the Lisbon Treaty).
The Damian McBride smear revelations left her "sickened" according to the Yorkshire Post:
"My stepdaughter Rachel said to me: ‘How could they do that to people like David Cameron and his wife Samantha when they had recently lost their son Ivan? What kind of people think it would be a good idea to smear them?' I was sickened by that – that is not the Labour Party that I joined all those years ago… Quite simply I have had it with New Labour."
Mahon continues to be active in left-wing politics, particularly through the Stop the War Coalition and CND. She is a Distinguished Supporter of the British Humanist Association and an Honorary Associate of the National Secular Society. The No2EU campaign reports she has decided to support them in the June 2009 European Parliament election.
She was formerly married to John Gledhill; the couple had two sons, Kris (a barrister, currently an academic at the University of Auckland Law School in New Zealand) and Kurt (who lives with his family in Yorkshire). The couple divorced in the 1970s and she married Tony Mahon.