[citation needed] In 1965, Quinn joined the Labour Party, working on Michael O'Leary's successful campaign in Dublin North-Central.

In the following years, Quinn was a leading student radical in UCD demanding reform of the university's structures and the old fashioned architectural course that then prevailed.

The party organisation was largely moribund since the 1969 general election, as Browne had been ill and little work had been done locally.

Following the 1973 election, Quinn began to rebuild the Labour Party in Dublin South-East with his mainly youthful supporters.

In July 1993, Quinn adopted a successful employment policy called the Back to Work Allowance, which targeted the long-term unemployed.

He also introduced the Community Employment Programme to provide activity and involvement for unemployed workers in 1994, which proved to be particularly successful.

Quinn was seen as a moderniser in economic terms, but who tried and failed to close the Irish Steel company in Haulbowline, County Cork.

Nevertheless, it was in August 1994, while Quinn and Fianna Fáil's Bertie Ahern were economic ministers, that the Irish economy was first described as the "Celtic Tiger".

Quinn, along with many of his Labour cabinet colleagues, strove unsuccessfully to keep the Fianna Fáil–Labour government together during the Father Brendan Smyth crisis in November 1994.

The following year, Quinn became Minister for Finance in the Rainbow coalition of Fine Gael, Labour and Democratic Left.

[2] He quickly proved his competence, dispelling opposition jibes and stock market fears about a social democratic minister holding the sensitive finance portfolio.

Many other ministers of the Labour Party were under significant pressure from the media (particularly the Irish Independent) concerning allegations of cronyism ("jobs for the boys") and abusing the privileges of office.

He used his years of leadership to develop a strong policy platform, publishing a Spatial Strategy for future development of the country, promoting universal access to health insurance, advocating reform of the Garda Síochána, and arguing for closer European integration.

Quinn fought that election on an independent platform, although he indicated a preference to enter government with Fine Gael, with which he had served with in the Rainbow coalition era.

This underestimated the attraction for the electorate of the outgoing Ahern Government, which had enjoyed extraordinary economic growth and prosperity.

When Rabbitte resigned as party leader in 2007, Quinn supported the successful candidacy of Eamon Gilmore.

His public support of Gilmore, where he also brought the endorsement of all the Dublin City Councillors in his area, was seen as instrumental in discouraging other candidates from entering the race.

Quinn caused anger and controversy when he refused to give up his minister's pension worth €41,656 while sitting as a TD in 2009.

Quinn became Labour Party spokesperson on Education and Science as a member of Eamon Gilmore's front bench in September 2007.

Quinn contributed to the successful second referendum on the Lisbon Treaty in September 2009 and continued to be an office holder with the Party of European Socialists.

[9] In July 2011, Quinn had again refused to rule out the return of college fees as he acknowledged the funding crisis in the higher education sector.

I have said to Brendan Howlin that I will deliver.”[10] In October 2012, Quinn announced the phasing out of the current Junior Certificate programme over the next eight years, to be replaced by a school-based model of continuous assessment.

[14] On 29 January 2013, Quinn launched Ireland's first national plan to tackle bullying in schools, including cyberbullying.

[15][16] Since 2016, Quinn has sat on various boards, including as chairperson of the Irish Architectural Archive (2020–2023), and as a director of the Institute of International and European Affairs.