After the death of president and longtime HDZ leader Franjo Tuđman, Kosor supported Ivo Sanader's successful party leadership bid in 2000.

After the abrupt resignation of Sanader, Kosor managed to form a functioning parliamentary majority and was approved to her new post as prime minister in July 2009, also becoming leader of her party.

As prime minister, Kosor failed to commit to structural reforms although she managed to prevent the country's budgetary meltdown with two budget revisions and the introduction of new taxes as a response to the ongoing economic crisis.

This uncompromising stance, along with the new criminal code passed before her term began, opened the door to unprecedented efforts to combat corruption.

This resulted in arrests of numerous influential business-people and politicians from across the political spectrum, although most of them were members of HDZ, which severely damaged the party's reputation.

[5] Her childhood friends describe her as a pretty, smart, and sociable girl that loved poetry and wrote songs.

[9][10] The Opposition was not pleased with this development calling Sanader a coward and Kosor his puppet saying that an early general election was necessary.

In the first month of her term Kosor, faced with a huge deficit and high unemployment, introduced an emergency budget aimed to reduce spending and the national debt.

[13] Indeed, the government's handling of finances was unpopular among the public resulting in the Prime Minister's dismal approval rating of 32% by the end of her first month.

In the last quarter of 2009, many public officials, as well as members of the boards of various government agencies, became suspected of participating in corrupt activities.

An unprecedented number[quantify] of officials were detained and arrested under these allegations which resulted in both praise and criticism of Kosor's government.

[citation needed] The accusations grew louder as more and more corruption affairs were tied with the former prime minister, Ivo Sanader.

He accused Kosor and the members of the HDZ Presidency of failed leadership citing Andrija Hebrang's poor result in the first round of the presidential election held just a week earlier.

[16] Most political pundits, as well as the majority of the public, believed the true reason of Sanader's surprise return was fear that he will eventually be tied with the numerous corruption scandals which have emerged since he left office.

Consumer spending reduced drastically compared to record 2007 levels, causing widespread problems in the trade as well as transport industries.

[19] The Constitutional Court ultimately declared the referendum issue moot, but ordered the government not to subject any changes to the labor law in the following year.

On 9 December 2011, Prime Minister Kosor and President Ivo Josipović signed EU Accession Treaty in Brussels.

[39] Kosor continued as an independent, considerably more liberal, politician and eventually formed a deputy club with two Croatian Civic Party MPs.

She continued to criticize Tomislav Karamarko and Kolinda Grabar-Kitarović as well as Bridge of Independent Lists party for their indecisiveness.

[53] In August 2016, Kosor stated that yelling Za dom spremni was "an insult to war veterans and their friends who gave their lives for a democratic, independent, and free Croatia".

[56] In 2017, she published a book which included texts she wrote on her blog and columns written for the Slovenian leftist newspaper Dnevnik in the period from 2015 to 2017 in which she commented on Croatian interior and foreign politics.

[57] Jadranka Kosor was married twice; between 1971 and 1981 to Hrvoje Markul, an editor of the HTV Entertainment Program, and between 1984 and 1993 to Ivo Škopljanac, the radio host.