A grandson of Irish revolutionary Eoin MacNeill, McDowell was a founding member of the Progressive Democrats in the mid-1980s. On three occasions he was elected as a Teachta Dála (TD) for the Dublin South-East constituency, serving in the 25th Dáil (1987–89), the 27th Dáil (1992–97), and the 29th Dáil (2002–07). He lost his Dáil seat at the general elections of 1989, 1997, and 2007.
McDowell led the Progressive Democrats to a disastrous performance in the 2007 general election, in which the party lost six of its eight seats in Dáil Éireann, including his own. After conceding his seat to John Gormley at the RDS count centre in Dublin, McDowell abruptly resigned as party leader and announced his immediate retirement from public life. He has since resumed his private legal career. He returned to politics in 2016 and was elected to Seanad Éireann on the National University of Ireland panel.
Born in Dublin, he was educated at the Jesuit school Gonzaga College, then at University College Dublin where he became auditor of the UCD Law Society. He later attended the King's Inns in Dublin where he achieved the Barrister-at-Law degree in 1974. McDowell was a junior counsel on the legal team that defended the murderer Malcolm MacArthur in the notorious GUBU case. In 2002, McDowell excused himself from considering MacArthur's parole report, to avoid any possible conflict of interest arising from this representation. He was appointed a Senior Counsel in 1987. He became involved in politics, initially supporting Fine Gael. When Desmond O'Malley was expelled from Fianna Fáil in 1985, McDowell immediately wrote to him in support, and ended up becoming one of the people who helped him establish the Progressive Democrats. He is the husband of UCD accountancy Professor Niamh Brennan and brother of UCD economics lecturer Moore McDowell.
McDowell was one of 14 Progressive Democrat TDs elected to Dáil Éireann at the 1987 general election, the first election after the party was founded. He was elected for the Dublin South-East constituency. He lost his seat at the 1989 general election but was made chairman of the party. McDowell regained his seat at the 1992 general election but lost it again at the 1997 general election. At various times, he served as a member of the Progressive Democrats front bench in roles as spokesman for foreign affairs, Northern Ireland and finance. In July 1999, McDowell was appointed Attorney General of Ireland, a position he held until 2002. In 2000 he proposed changing the name of the party to the Radical Party.
Following the 2002 general election, McDowell regained his Dáil seat. This was the first time McDowell combined winning a Dáil seat with his party's entry into government. He was appointed to the cabinet as Minister for Justice, Equality and Law Reform. He was a strong opponent of the policies of Sinn Féin and the Provisional Irish Republican Army, and often took a harder line than his coalition partners, Fianna Fáil. He was named as Politician of the Year for 2004 in the Magill magazine annual awards.