Janša graduated from the Faculty of Social Sciences of the University of Ljubljana with a degree in Defence Studies in 1982, and became a trainee in the Defence Secretariate of the Socialist Republic of Slovenia.

In 1983, Janša wrote the first of his dissident articles about the nature of the Yugoslav People’s Army (JNA). In the late 1980s, as Slovenia was introducing democratic reforms and gradually lifting restrictions on freedom of speech, Janša wrote several articles criticising the Yugoslav People’s Army in Mladina magazine (published by the League of Socialist Youth of Slovenia). As a result, his re-election as president of the Committee was blocked in 1984, and in 1985 his passport was withdrawn. He said that he made over 250 job applications in the following year without success, and was unable to secure publication of some articles. Other articles are documented in COBISS In this period he earned his living writing computer programs and acting as a mountaineering guide. Liberalization in the succeeding years allowed him to get work as secretary of the Journal for the Criticism of Science (1986) and later to begin publishing again in Mladina magazine.

In 1987, Janša was approached by the family of the late politician Stane Kavčič, who had been the most important exponent of the reformist fraction in the Slovenian Communist Party in the late 1960s, and prime minister of Slovenia between 1967 and 1972; he was asked to edit the manuscript of Kavčič’s diaries. Janša edited the volume together with Igor Bavčar. The publication of the book was part of the political project of Niko Kavčič, former banker and prominent member of the reformist wing of the Communist Party, to establish a new Slovenian left wing political formation that would challenge the hardliners within the Communist Party.

On 30 May 1988, he was arrested together with three other Mladina journalists and a staff sergeant of the Yugoslav Army, Ivan Borštner. They were tried in a military court on charges of exposing military secrets, and given prison sentences. The trial was conducted in camera, with no legal representation for the accused, and in Serbo-Croatian (the official language of the Yugoslav Army) rather than in Slovene.

The philosopher Slavoj Žižek, who at the time also worked as a columnist for Mladina, suggested that Janša was arrested because of his critical articles on the Yugoslav Army, and because the army wanted to prevent his election as president of the League of the Socialist Youth of Slovenia. As a consequence of his arrest, he could not run for the position; nevertheless, the leadership of the organization decided to carry on with the elections despite Janša’s arrest. In June 1988, Jožef Školč was elected as president of the League instead of Janša.

In 1989, Janša was involved in the founding of one of the first opposition parties in Slovenia, the Slovenian Democratic Union (SDZ) and became its first vice-president, and later president of the Party Council. Following the first free elections in May 1990 he became the Minister of Defence in Lojze Peterle’s cabinet, a position he held during the Slovenian war for independence in June and July 1991. Together with the Minister of Interior Igor Bavčar, Janša was the main organizer of Slovenia’s strategy against the Yugoslav People’s Army.

The largest and most notable Roman Catholic newspaper Družina and Janša have both claimed that the very few individuals who managed to survive the Kočevski Rog massacre included Janša’s father, although the story of the actual survivor France Dejak, which was told in 1989 for the first time in Mladina, was re-told in details as if it has been experienced by Janša’s father.

In 1992, when the Slovenian Democratic Union broke into a liberal and a conservative wing, the leaders of the liberal fraction wanted to propose Janša as the compromise president of the party, but he refused the offer. After the party’s final breakdown, he joined the Social Democratic Party of Slovenia (now called Slovenian Democratic Party) and remained Defence Minister in the center-left coalition government of Janez Drnovšek until March 1994. In May 1993, he was elected president of the Social Democratic Party of Slovenia with the support of Jože Pučnik, the party’s previous leader, and was re-elected in 1995, 1999, 2001, 2005 and 2009.

Janša has published several books, the two of which are Premiki (“Manoeuvres”, published in 1992 and subsequently translated into English under the title “The Making of the Slovenian State”) and Okopi (“Barricades”, 1994), in which he exposes his personal views on the problems of Slovenia’s transition from Communism to a parliamentary democracy. In both books, but particularly in Okopi, Janša criticized the then president of Slovenia Milan Kučan of interfering in daily politics using the informal influence he had gained as the last chairman of the Communist Party of Slovenia. He published second edition of the same book: Dvajset let pozneje Okopi with some additional documents and personal views.

In March 1994, Janša was dismissed by Prime Minister Janez Drnovšek as a consequence of the Smolnikar scandal (also known as Depala Vas scandal). The scandal began when three military intelligence servicemen allegedly brutally arrested a civilian, hired by the Ministry of the Interior for espionage. Janša was never accused of direct responsibility for this action, but his public defence of the military agents who carried out the arrest outraged the left wing sector of public opinion. In response, Drnovšek dismissed Janša and removed the Social Democratic Party from the ruling coalition. The official charges against the military servicemen involved were later dismissed, but the issue remains a point of controversy. Janša used the parliamentary debate on his dismissal to criticize his former allies, Drnovšek and President Milan Kučan, whom he accused of abusing his informal connections for subversive political actions. Janša’s dismissal caused a great stir in the public opinion, including mass demonstrations in his support. In local elections the same year, the Social Democratic Party experienced a significant boost in popularity, becoming the main opposition force, and in the 1996 parliamentary elections Janša’s party rose from around 3.5% to more than 16%, becoming the third largest political party in the country.

During his time in opposition, Janša supported the government’s efforts for the integration into EU and NATO. Between 2002 and 2004, he re-established cordial relations with now-President Drnovšek: in 2003, Drnovšek headed a round table on Slovenia’s future based on Janša’s recommendations.

It was also for the first time after 1992 that the president and the prime minister had represented opposing political factions for more than a few months. The relationship between Drnovšek and the government quickly became tense. After the landslide victory of the opposition candidate Danilo Türk in the 2007 presidential election, Janša filed a Motion of Confidence in the government on 15 November 2007, stating that the opposition’s criticism was interfering with the government’s work during Slovenia’s presidency over the European Union. The government won the vote, held on 19 November, with 51 votes supporting it and 33 opposing it. In the speech delivered after the vote, Janša announced, among other, an intensification of the fight against financial criminality and the illegal concentration of capital in the hands of single powerful managers, to whom he referred as tycoons. In the following months, the Slovenian police and public prosecution launched a full-scale investigation against some of the biggest companies in the country, namely against the Laško Brewery Concern.

Allegations were made against Janez Janša that he tried to subordinate Slovenian media. On 1 September 2008, some three weeks before the Slovenian parliamentary elections, allegations were made in Finnish TV in a documentary broadcast by the Finnish national broadcasting company YLE that Janša had received bribes from the Finnish defense company Patria (73.2% of which is the property of the Finnish government) in the so-called Patria case. Janša rejected all accusations as a media conspiracy concocted by left-wing Slovenian journalists, and demanded YLE to provide evidence or to retract the story. Janša’s naming of individual journalists, including some of those behind the 2007 Petition Against Political Pressure on Slovenian Journalists, and the perceived use of diplomatic channels in an attempt to coerce the Finnish government into interfering with YLE editorial policy, drew criticism from media freedom organizations, such as the International Press Institute and European branch of International Federation of Journalists whose representative, Aidan White, IFJ general secretary, said “The (Janša’s) government is distorting the facts, failing to tell Slovenians the truth and trying to pull the wool over the eyes of the European public about its attitude to media”.

In the November 2008 election, Janša’s party was placed second and he was replaced as prime minister by Borut Pahor, the Social Democrat leader. After the onset of Financial crisis of 2007–2010 and European sovereign-debt crisis, the left-wing coalition that replaced Janša’s government in 2008 elections, had to face the consequences of the 2005–2008 over-borrowing; however, all the attempts to implement reforms that would help towards economic recovery were met by student protesters, led by a student who later became a member of Janez Janša’s SDS, and by the trade unions. The proposed reforms were postponed on a referendum.

In 2008, it was reported by the newspaper Mladina that Janez Janša copied a speech by Tony Blair, the prime minister of the United Kingdom. It was used in 2006 for the ceremony on the 15th anniversary of the Slovenian declaration of independence. His office responded with the claim that it was not copied but similar to Blair’s speech, and that this were only a few phrases often used for such occasions. A few of these sentences were proclaimed the Spade of the Year by the newspaper Večer in 2006; the award is given annually to the best publicly expressed thought in Slovenia.

Since July 2009, Janša has been married to Urška Bačovnik (MD) from Velenje. The two had been dating since 2006. In August 2011, their son Črtomir was born. Their second son, Jakob, was born in August 2013.

In December 2011 Janša’s party won the second place in the Slovenian parliamentary elections. Since the prime minister-designate of the first-placed party, Positive Slovenia, Zoran Janković failed to secure himself enough votes in the National Assembly, and Danilo Türk, the President of Slovenia, declined to propose Janša as prime minister because Janša had been charged in the Patria bribery case, Janša was proposed as prime minister by the coalition of the parties SDS, SLS, DeSUS, NSi, and the newly formed Gregor Virant’s Civic List on 25 January 2012. On 28 January he became prime minister-designate. His cabinet was confirmed on 10 February, and Janša became the new prime minister with a handover from Pahor on the same day. On 13 February the President received the new Government and wished them luck. Both parties agreed that good cooperation is crucial for success.

In January 2013, the 2012–2013 Investigation Report on the parliamentary parties’ leaders by Commission for the Prevention of Corruption of the Republic of Slovenia revealed that Janez Janša and Zoran Janković systematically and repeatedly violated the law by failing to properly report their assets. It revealed his purchase of one of the real-estate was indirectly co-funded by a construction firm, a major government contractor. It showed that his use of funds in the amount of at least 200.000 EUR, coming from unknown origin, exceeded both his income and savings.

Immediately after the release of the report, Civic List issued an ultimatum to Janša’s party to find another party member to serve as a new PM. Since Janša was ignoring the report and his party didn’t offer any replacement for him, all three coalition parties and their leaders left the government within weeks and were subjected to ad hominem attacks by Janez Janša who accused the SLS’s leader Radovan Žerjav of being “the worst (economics) minister in history of Slovenia”, while the leader of the Civic List Gregor Virant has been mocked by Janša as engaging in “virantovanje” (a word game on kurentovanje, a Slovenian carnival festival). On 27 February 2013, Janša’s government fell, following a vote of no confidence over allegations of corruption and an unpopular austerity programme in the midst of the country’s recession. Gregor Virant welcomed the outcome of the vote, stating that it will enable Slovenia to move forward, either to form a new government or to call for an early election. Janša took over Ministry of Finance on 1st of February.

On 5 June 2013, the District Court in Ljubljana ruled that Janša and two others had sought about €2m in commission from a Finnish firm, Patria, to help it win a military supply contract in 2006 (Patria case). Janša was sentenced to two years while Tone Krkovič and Ivan Črnkovič, his co-defendants, were each sentenced to 22 months in prison. All three were also fined €37,000 each. Janša has denied the accusations, claiming the whole process is politically motivated. The following day, the Minister of Justice, Senko Pličanič, emphasised that the court ruling was not yet binding and therefore Janša was still presumed innocent.

After the Constitutional Court of Slovenia with the majority of votes dismissed Janša’s appeal due to him not having exhausted every other legal means available to him, on 20 June 2014 Janša started serving his prison term in Dob Prison, the largest Slovene prison. He was escorted there by about 3,000 supporters. The influential German centre-right wing newspaper Frankfurter Allgemeine Zeitung reported the following day that the domestic Slovene and the international law experts recognised large violations of Janša’s rights in the court case. The case is to be reviewed by the Supreme Court, but this does not postpone the execution of the sentence that started just three weeks before the parliamentary election. Former constitutional judges criticised the decision of the Constitutional Court for being based on formalities instead of on the content, and commented that a large legal inconsistency in the process was discovered only in front of the Constitutional Court and that it will prevent the Supreme Court from not overturning the judgement. On 12 December 2014 Janša was temporarily released from the prison pending the review of the case by the Constitutional Court. The conviction was unanimously overturned by the Constitutional Court on 23 April 2015.

In the early election on 3 June 2018, Janez Janša was re-elected as a deputy. He was elected in the electoral district of Grosuplje and received 7,020 votes or 39.3%, the largest share of all candidates in the country. The Slovenian Democratic Party (SDS) won the election by receiving 24.92% of votes and gaining 25 seats out of 90 in the National Assembly.

Following the resignation of Marjan Šarec as prime minister, Janša was elected as prime minister-designate on 3 March 2020, to form the 14th Government of Slovenia. He was sworn in on 13 March 2020.

On November 4, 2020, the day after the United States presidential election, Janša congratulated Donald Trump on his alleged reelection; he remained the only world leader to have done so when news organizations instead called the election for Joe Biden on November 7.`