He entered the Department of Foreign Affairs, being appointed a Third secretary in 1974 and became a First Secretary in 1977. Later recruited by TaoiseachCharles Haughey, he worked for the Fianna Fáil party thereafter, serving under three Fianna Fáil leaders as Director of Research, Policy and Special Advisor on Northern Ireland where he was involved in discussions between the nationalist parties and the Irish Government and met regularly with intermediary Father Alec Reid. Mansergh was a key member of the team which formed the Fianna Fáil–Labour Party coalition in 1992 and was also involved in the formation of the Fianna Fáil–Progressive Democrats coalition in 1997. As a senior adviser to successive Taoisigh, Mansergh has played a key role in the Northern Ireland peace process over the last twenty years. He ran for Fianna Fáil as a Dáil candidate in the Tipperary South constituency at the 2002 general election but failed to be elected with 14.2% of the poll. However, Mansergh was elected to the 22nd Seanad by the Agricultural Panel in July of that year. At the 2007 general election he again ran for Fianna Fáil as a Dáil candidate in the Tipperary South constituency, this time being elected with 15.7% of the poll. He was formerly a member of the Irish Council of State. Until 2006 he wrote a weekly column for The Irish Times, but resigned because of the upcoming general election. In May 2008, he was appointed as Minister of State at the Department of Finance with special responsibility for the Office of Public Works, as well as Minister of State at the Department of Arts, Sport and Tourism with special responsibility for the Arts. In January 2009, he offered to quit his junior ministry post to save money and called on people to retain their Celtic Tiger style optimism and self-respect. He said: "We're not going to get anywhere by completely throwing overboard our self respect. We have achieved a tremendous amount in the past 20 years – they were the best 20 years in our history. There will be cycles – we rose very high and we are where we are now. We have to work our way out of this intelligently". He lost his seat at the 2011 general election. He is vice-chair of the government's Expert Advisory Group on the Decade of Centenaries. He was elected a member of the Royal Irish Academy in May 2018. He has been a frequent contributor to The Irish Catholic.
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Mansergh has been a strong supporter of former Taoiseach Bertie Ahern, whose financial affairs were investigated by the Mahon Tribunal. He has been accused by some commentators of being insulting, condescending and petulant to opposition politicians. On an RTÉ radio show Morning Ireland in February 2008, Mansergh reassured listeners that Ahern's difficulties were no more than a spot of "inflight turbulence," with a safe landing in sight. When Fine Gael's tribunal expert, Senator Eugene Regan dissented, Mansergh became quite agitated, questioning why Regan wanted to question Ahern's finances declaring to Regan that: "You should have respect for your betters!"
