Born in Dublin but raised in Limerick, Cox was educated at Ardscoil Rís in Limerick, the University of Limerick and Trinity College, Dublin. Cox is married to Cathy, and lived at Ashboro, Shanakiel in the Cork northside suburb of Sunday's Well for 16 years.
Cox first came to prominence as a journalist, then a presenter with RTÉ's Today Tonight, a four-nights-a-week current affairs programme which dominated the Irish television schedules in the 1980s. He stood as a Fianna Fáil candidate at the 1979 local elections. Cox left the programme to become a political candidate. He was elected an MEP in 1989 for the constituency of Munster, representing the Progressive Democrats (PDs). At the 1992 general election he was also elected to Dáil Éireann as a Teachta Dála (TD) for Cork South–Central.
Following Desmond O'Malley's retirement from the party leadership in 1993, Cox stood for election to the post but was beaten by Mary Harney. Cox became deputy leader.
Cox left the PDs in 1994 in a dispute over his seat as an MEP. It was expected that Cox would not contest his seat in the 1994 European elections, with O'Malley, who had a large Munster base in Limerick city and County Limerick, becoming the party candidate. However Cox then decided to contest the seat as an independent, beating O'Malley, the PD candidate. On being elected, he resigned his Dáil seat and a by-election was held on 10 November 1994, which was won by Fine Gael.
He was elected president of the ELDR group in the European Parliament in 1998 and was unanimously re-elected Group President in June 1999 following his re-election as an MEP at the 1999 European Parliament election. He resigned this post when he became President of the European Parliament on 15 January 2002 in accordance with an agreement between the European People's Party (Christian Democrats) and the ELDR groups at the start of the term (in the customary two-way split of the five-year Presidency of the European Parliament). He succeeded the Frenchwoman Nicole Fontaine. At his first press conference following his election as president he spoke positively of direct talks between the Cypriot President Glafcos Clerides and Turkish Cypriot President Rauf Denktaş.
In July 2003 Cox took a personal telephone apology from Silvio Berlusconi after Berlusconi managed to offend several MEPs. The controversy arose after Berlusconi compared a German MEP to a Nazi concentration camp commandant.
On 20 May 2004 he was awarded the Charlemagne Prize (Karlspreis) for his achievements with regard to the enlargement of the European Union and for his work in promoting greater EU democratisation.
Cox did not contest the 2004 elections to the European parliament. The Christian Democrats (European People's Party – EPP) and Socialist Groups agreed at the customary two-way split of the Presidency of the European Parliament. Josep Borrell Fontelles, a Spanish Socialist, assumed the Presidency on 20 July 2004, holding it until 15 January 2007.
Cox is a member of the Comite d'Honneur of the Institute of European Affairs. In 2006 he was elected President of European Movement, an international pro-European lobby association. In June 2009 Pat Cox temporarily stepped down as President and took over the position of the campaign director for the pro-Lisbon treaty initiative Ireland for Europe. He resigned as president of European Movement in May 2013.
Also in 2009, Cox co-founded the European Privacy Association.
On 15 September 2010 Cox supported the new initiative Spinelli Group, which was founded to reinvigorate the drive toward federalisation of the European Union (EU). Other prominent supporters include Jacques Delors, Daniel Cohn-Bendit, Guy Verhofstadt, Andrew Duff and Elmar Brok.
He is a Grand Cross of the Order of the Star of Romania and a Knight Grand Cross of the Order of Merit of the Italian Republic. He received the 2004 Charlemagne Prize for his efforts in the eastward expansion of the European Union.
Since June 2012 Cox and Aleksander Kwaśniewski have led a European Parliament monitoring mission in Ukraine to monitor the criminal cases against Yulia Tymoshenko, Yuriy Lutsenko and Valeriy Ivaschenko.
Since 1 January 2015, he has been the president of the Jean Monnet Foundation for Europe.
In June 2011 Irish media reported that Cox, who had previously said he wanted to stand as an independent candidate in 2011's Irish presidential election, was seeking to join Fine Gael to get the party's nomination. Fine Gael's national executive on 16 June 2011 approved his application to join the party's St Luke's branch in Cork. In July 2011 Gay Mitchell became the Fine Gael candidate. Before that, Cox was "pleased" to help prepare Fine Gael's first-100-day strategy after it won the Irish general election, 2011.