Mary Harney was born on 11 March 1953 in Ballinasloe, County Galway. Her parents were both farmers, but she was mostly brought up in Newcastle, County Dublin. She was educated in Clondalkin, Inchicore and Trinity College, Dublin. She came to the attention of Fianna Fáil leader Jack Lynch in her role as auditor of the university's primary debating society. She stood for the Dáil in the Dublin Soth-East constituency in the 1977 general election, however she was unsuccessful. Jack Lynch became Taoiseach and she was one of his nominees to Seanad Éireann.
In 1981 she contested the election again and was successful. She has retained her seat at every election since then. She faced a number of problems, like many others, after the election of Charles Haughey as leader of Fianna Fáil. She was left Fianna Fáil after defying the party whip by voting in favour of the Anglo-Irish Agreement in 1985. She went on the become a founder-member of the Progressive Democrats with Desmond O'Malley later that year.
When Fianna Fáil formed a coalition government with the Progressive Democrats in 1989, Harney became Minister of State at the Department of the environment. In 1993 she became leader of the Progressive Democrats, the first woman to lead a political party in Dáil Éireann. In 1997 the party formed a coalition with Fianna Fáil. Harney became Tánaiste, the first woman to hold that office. She also became Minister for Enterprise, Trade and Employment. She has retained that position since 1997.