Séamus Brennan was born on 16 February 1948 in Galway. He was educated in Galway city at University College Galway and University College Dublin. He found an interest in politics during his teens when he canvassed for Fianna Fáil during elections. In the early 1970's he succeeded Tommy Mullins as Secretary General of Fianna Fáil. He revamped the party structure with the setting up of a national executive. Brennan was a key member in the 1977 general election victory. He helped modernise Irish elections due to the use of marketing strategies and opinion polls. Following the return of Fianna Fáil to power he was elected to Seanad Éireann.
In 1979 he supported George Colley in the leadership contest. When Charles Haughey won a new Secretary General of the party was appointed. In 1981 Brennan was elected to Dáil Éireann and has been returned ever since then. In the early 1980's he was aprominent member of the Gang of 22 who tried to wrestle control of the party from Charles Haughey. It was expected that Brennan would join the Progressive Democrats when thay were founded in 1985 but instead he remained with Fianna Fáil.
In 1987 Brennan was rewarded with a Trade and Marketing portfolio. This was followed with several other Cabinet positions including Minister for Tourism and Transport (1989-1991), Minister for Tourism, Transport & Communications (1991-1992) , Minister for Education (1992-1993) and Minister for Commerce & Technology (1993-1994). On Fianna Fáil's return to power in 1997 Brennan was appointed Government Chief Whip and Minister for State at the Departments of An Taoiseach and Defence. Following the 2002 general election victory Brennan was rewarded with a full Cabinet position by becoming Minister for Transport.