After finishing the Nova Gorica High School in 1983, Pahor enrolled in the University of Ljubljana, where he studied public policy and political science at the Faculty of Sociology, Political Science and Journalism (FSPN, now known as Faculty of Social Sciences, FDV). He graduated in 1987 with a thesis on peace negotiations between members of the Non-Aligned Movement. His B.A. thesis was awarded the Student Prešeren Award, the highest academic award for students in Slovenia. According to the Slovenian press, Pahor worked as a male model to pay for his university studies.

In 1987, he ran for the Presidency of University Section of the Alliance of the Socialist Youth of Slovenia. This internal election was important, as it was the first election in Yugoslavia organized entirely according to democratic principles. In the election, in which the members could freely choose between two antagonistic teams, Pahor’s team lost to a more liberal faction.

In 1989, Pahor co-founded and chaired the Democratic Forum, a youth section within the Slovenian Communist Party established as a counter-force to the Alliance of Socialist Youth, which was now already openly opposing the communists’ policies. The same year, he was appointed to the Central Committee of the League of Communists of Slovenia, thus becoming the youngest member of this body in its history. In 1990, he participated in the Slovenian delegation at the last Congress of the League of Communists of Yugoslavia in Belgrade.

As a consequence, the Youth Alliance emancipated from the control of the Communist Party: a process that resulted in the formation of the Liberal Democratic Party in 1990. Due to this shift, Pahor continued his political career in the main apparatus of the Communist Party. He rose to prominence in the late 1980s, when he became one of the strongest supporters of the reformist wing of the Communist Party, led by Milan Kučan and Ciril Ribičič.

In the first free elections in Slovenia in April 1990, in which the communists were defeated by the Democratic Opposition of Slovenia (DEMOS), Pahor was elected in the Slovenian Parliament on the list of the League of Communists – Party of Democratic Reform. Together with Milan Balažic, Pahor emerged as the leader of the pro-reformist wing of the party, which advocated a clear cut with the communist past and a full-fledged acceptance of free-market economy; they even went so far to propose the merger of the party with Jože Pučnik’s Social Democratic Party of Slovenia. As the party continued to lose support during the whole 1990s, falling under 10% of the popular vote in 1996, Pahor’s positions grew in strength. In 1997, he was elected as its president on a Third way-centrist platform.

Pahor won the second round with about 53 percent of the vote, with 99.9 percent of the votes counted, according to the Election Commission, while his opponent, Marjan Šarec, had 47 percent. Turnout was about 42 percent, according to preliminary figures, the lowest for a presidential election since Slovenia became an independent country in 1991.

In 1997, he was involved in the attempt of creating a common left-wing government between Pahor’s United List of Social Democrats, the Liberal Democracy of Slovenia, the Slovenian National Party, and the Pensioner’s Party. Pahor was proposed as Minister of Foreign Affairs in this left-wing coalition government, but the proposal failed to gain a majority in the parliament. Instead, the Liberal Democracy of Slovenia formed a coalition with the conservative Slovenian People’s Party, based on a centrist platform, which ruled until 2000. Pahor’s Social Democratic party remained in opposition, although it supported the government in several key decisions.

In 2000, Pahor led his party in the coalition with the Liberal Democracy of Slovenia led by Janez Drnovšek. Pahor was elected speaker of the Slovenian National Assembly (the lower house of the Slovenian Parliament). This was his first important institutional office. During this period, he distinguished himself with a moderate and non-partisan behaviour, which gained him the respect of large sectors of the centre-right opposition.

Pahor has tried taking a more active role as president, which is mostly a ceremonial post. This has involved meeting youth at publicized events in the presidential offices in Ljubljana as well as appearing as a speaker in some major events in the country. He also met Vladimir Putin, whom he encouraged to try to resolve the Ukrainian conflict, and suggested a Trump-Putin meeting in Ljubljana, which has previously served as a venue for such occasion in 2001.

In June 2004, he was elected as member of the European Parliament, where was a member of the Socialist group. He served on Parliament’s Budgetary Control committee and the Constitutional Committee during the period of the rejection of the Constitutional treaty by France and the Netherlands and the negotiation of the Lisbon Treaty, supporting the Parliament’s line on this (Richard Corbett and Inigo Mendez de Vigo report). In October 2004, the centre-left coalition in Slovenia lost to the liberal-conservative Slovenian Democratic Party and its conservative allies. In the first years of Janez Janša’s centre-right government, Pahor openly polemized with Anton Rop, the leader of the Liberal Democracy of Slovenia, over the opposition strategy towards the government. In the polemics, which soon became known to the public as the “Dear Tone, Dear Borut Discussion” (after the opening lines of the leaders’), Pahor opted for a more constructive opposition. In 2006, Pahor’s Social Democrats entered an agreement with the ruling coalition party for the collaboration in the economic reform policies.

Faced by the global economic crisis his government proposed economic reforms, but they were rejected by the opposition leader Janez Janša and by referenda in 2011. On the other hand, the voters voted in favour of an arbitration agreement with Croatia, aimed to solve the border dispute between the countries, emerging after the breakup of Yugoslavia.

Tensions between the coalition partners reached the summit in 2011, when two parties, DeSUS in April and Zares in July, left the government. The opposition has accused the government of corruption and mishandling the economy. Faced with the loss of several ministers and falling public support, Pahor asked the Parliament for a motion of confidence. On 20 September, the Parliament voted 51–36 against the motion, resulting in the fall of the government. After the vote, Pahor said: “I do not feel any bitterness. I have full faith in our people and the future of Slovenia.”

On 1 December 2011, several clips of the recordings of closed sessions of the Government of Slovenia during Borut Pahor’s term were published on the video-sharing website YouTube.

On 4 December 2011 elections, under Pahor rule the party went from 29 to 10 (losing 19) seats at the early National Assembly election in comparison to the 2008 elections, but Pahor expressed “great contentment” with the result and explained that the party won “more votes than he expected”. On 19 December 2011, while still in hospital due to otitis media, Pahor accepted the candidacy for the Speaker of the National Assembly after National Assembly election, but retracted it after two unsuccessful election rounds.

In June 2012, Pahor unsuccessfully ran for re-election as president of the Social Democrats. He was defeated by Igor Lukšič by a narrow margin. At the same party congress, Pahor announced he would run for President of Slovenia. A few days later, the party and its new president officially supported Pahor’s candidacy for president. In September, the Civic List, a centrist party in the Slovenian center-right government coalition, also officially supported Pahor’s candidacy for president.