O’Rourke is educated at St. Peter’s in Athlone, Loreto Bray Convent in County Wicklow, University College Dublin and St. Patrick’s College, Maynooth. She works as a secondary school teacher before beginning her political career.
O’Rourke begins her political career in local politics, serving on Athlone Urban District Council between 1974 and 1987 and on Westmeath County Council between 1979 and 1987. She is elected to Seanad Éireann in 1981 as a Senator for the Cultural and Educational Panel. She stands unsuccessfully for the Dáil at the February 1982 Irish general election, but is subsequently re-elected to the Seanad. At the November 1982 Irish general election, she is first elected to Dáil Éireann as a Fianna Fáil TD for the Longford–Westmeath constituency, and from 1992 for the new Westmeath constituency.
In 1987, O’Rourke is appointed Minister for Education by Charles Haughey. She and her brother, Brian Lenihan Snr, become the first brother and sister in Irish history to serve in the same cabinet. In the November 1991 cabinet reshuffle, she becomes Minister for Health. In February 1992, Charles Haughey resigns as Taoiseach and Fianna Fáil leader and she contests the subsequent leadership election along with Michael Woods and Albert Reynolds. Reynolds wins the election and she is subsequently dropped from her ministerial position, but is appointed to a junior ministry as Minister of State for Labour Affairs at the Departments of Industry and Commerce, and later Enterprise and Employment.
In 1994, Bertie Ahern becomes party leader and he appoints O’Rourke as deputy leader of Fianna Fáil, serving in the position until 2002. Following Ahern’s election as Taoiseach in June 1997, she becomes Minister for Public Enterprise, holding this position until she loses her Dáil seat at the 2002 Irish general election. This follows a vote management strategy from Fianna Fáil head office which restricts her from campaigning in her traditional areas around Kilbeggan, in an attempt to win 2 of the 3 seats in Westmeath. The loss of her Dáil seat is also attributed to her association with and the championing of, the privatisation of Telecom Éireann, which proves a financial disaster for many small investors, due to the share price falling radically, post privatisation. During this term as Minister, she also becomes the subject of public criticism by Ryanair chief executive Michael O’Leary. Following the loss of her Dáil seat, she is nominated to Seanad Éireann as a Senator by Taoiseach Bertie Ahern where she becomes Leader of the Seanad and leader of Fianna Fáil in the Seanad.
In January 2006, O’Rourke receives the party nomination to stand at the 2007 Irish general election. She narrowly defeats her nearest rival and Dáil election running mate, Kevin “Boxer” Moran of Athlone Town Council, causing a controversy when she thanks her election team for working “like blacks.” She is re-elected to the Dáil at the May 2007 Irish general election, with her highest ever vote.
In November 2008, during a march against the re-introduction of college fees, students from the Athlone Institute of Technology lay a funeral wreath at the door of O’Rourke’s constituency office. The card in the wreath states “Sincere sympathies on the death of free fees. We will remember this.” She describes the act as “heinous.” The wreath is placed there because she is not speaking at a rally against the fees.
In July 2010, O’Rourke concedes that she does not expect the party to be in power after the next general election. On RTÉ Radio‘s Today with Pat Kenny programme, she says the government is taking tough decisions to steer the country through the financial crisis and this will make it easy for the opposition. She says there is a general air of “crossness” within the Fianna Fáil party over their standing in the polls, but nobody is harboring leadership ambitions to challenge Brian Cowen.
In November 2010, O’Rourke says there is then more to unite her party and Fine Gael than to divide them. She points to the common approach of the two parties to Northern Ireland, Europe and the current financial crisis. In an address to the 1916–1921 Club in Dublin Castle, she says that most voters no longer defined themselves in terms of Civil War politics.
O’Rourke’s senior years lead her to often being referred to as the “Mammy of the Dáil.”
O’Rourke contests the 2011 Irish general election, but is defeated on the poll. She had been critical of former Taoiseach Brian Cowen, saying that he should have resigned after his infamous “congested” radio interview. She supports the attack on Cowen by her nephew, former Finance Minister Brian Lenihan Jnr, who says he is “disappointed” by Cowen’s performance and he had to provide the leadership when the Taoiseach did not.
As well as being a well-known politician, O’Rourke makes regular appearances in the media in a non-political capacity. She has been a contestant on RTÉ‘s reality series Celebrity Bainisteoir, as well as other shows such as Sex & Sensibility. She has guest presented Tonight with Vincent Browne.
In 2012, Just Mary: My Memoir is published. It wins the 2012 Irish Book Award in the “Listeners’ Choice” category.
O’Rourke comes from a strong political family, her father Patrick Lenihan serves as a TD for Longford–Westmeath from 1965 to 1970. Her brother Brian Lenihan is a senior government Minister and Tánaiste. Another brother, Paddy Lenihan, is a County Councillor in Roscommon, but resigns from Fianna Fáil in 1983 and becomes associated with Neil Blaney‘s Independent Fianna Fáil party. Two of her nephews, Brian Lenihan Jnr and Conor Lenihan, both sons of her brother Brian, serve as Ministers. Brian Lenihan Jnr is the Minister for Finance. Conor Lenihan is a Minister of State.
O’Rourke is widowed in January 2001, following the death of her husband, Enda. She has two sons. Aengus O’Rourke, her adopted son, runs for Athlone Town Council in 2009. The other son, Feargal O’Rourke, becomes Managing Partner of PricewaterhouseCoopers in Ireland in 2015 and is considered the “grand architect” of the Double Irish tax system, a major contributor to Ireland’s economic success in attracting U.S. multinationals to Ireland.