Massimov, of Tajik and Uyghur descent, graduated from the Republican Physics and Mathematics Boarding School in 1982 and studied Arabic in Russia.

Massimov was first appointed Prime Minister by President Nursultan Nazarbayev in January 2007, facing significant challenges during the Great Recession.

In September 2012, he was dismissed as Prime Minister and appointed chief of staff of the presidential office to readjust the power balance within the government.

He was reappointed Prime Minister in April 2014, where he managed further economic difficulties, including a 20% devaluation of the Kazakhstani tenge and falling oil prices, while also leading the 100 Concrete Steps modernization plan.

Massimov later became the head of the National Security Committee in September 2016,[6] a position he held until his dismissal by President Kassym-Jomart Tokayev following the civil unrest in January 2022.

[17] That same year, he defended his doctoral dissertation on the topic "Problems of the formation of industry of the Republic of Kazakhstan and ways to solve them (theory and practice)".

"[25] On 28 March 2002, in an article in Izvestia, Massimov announced that the Government of Kazakhstan planned to increase wheat exports to Iran from 100,000 to two million tons.

Massimov said, "I came to Israel with a clear message to the nation in Zion from the president, that Kazakhstan is a moderate Muslim state which is interested in being involved in the Middle East.

[27] Massimov also met with Israeli Prime Minister Ehud Olmert, who praised Kazakhstan for showing a "beautiful face of Islam.

A couple of months later, he opened his blog on the government's website, explaining the need to provide high-quality, interesting and useful information about the socio-economic situation in the country.

Massimov ordered to stabilize prices in Kazakhstan by instructing law enforcement agencies to ensure that the dollar exchange rate fluctuates within 150 tenge plus or minus 3%.

[33] The reforms and strategic interventions pursued by the government almost doubled the GDP per capita of Kazakhstan from 2008-2012, with economic growth rates reaching an average level of 5.5% per year.

[36] Massimov was directly involved in the preparation for the 2011 Asian Winter Games, by ordering to make the facilities accessible to ordinary citizens "so that they could improve their sports skills.

"[32] On 24 September 2012, Massimov announced his resignation from the post of the PM, which was accepted by President Nursultan Nazarbayev in which he explained "his work fell on the time of the strongest global financial and economic crisis, and our country came out with honor, this is a large part of the merit of the government and the prime minister.

The possible reason was due to the economic troubles under Akhmetov in the country which faced lower GDP growth and devaluation of the tenge by 20% in February 2014 with Massimov having a better experience in running the government for such issues.

[41] In June 2014, Massimov attended the presidential inauguration of Petro Poroshenko on the orders of President Nazarbayev as a way of normalizing Kazakhstan's relations with Ukraine.

[45] In response to the crisis, Massimov believed that without the intervention from the government, the economy would experience a default with its growth being restored along with an increase in credit and investment activity, creation of new jobs as well as the reduction of inflation.

[47] Court hearings lasted several months which resulted in the ex-head of the national company Talgat Ermegiyaev was sentenced to 14 years in June 2016 after being found guilty of embezzling 5.9 billion ₸.

In his address to the International Olympic Committee, he assured the IOC by saying:“We are a great opportunity to prove that developing countries can successfully host the Winter Games.

[57][3] After being relieved from his post as the PM, Massimov, while serving as the Head of the Presidential Administration, was also the Acting State Secretary of Kazakhstan from 21 January 2014 to 2 April 2014.

[62] According to the Prosecutor General of Kazakhstan, the details of the criminal case and the court hearing were classified as containing state secrets, and information concerning foreign policy, counterintelligence and operational-search activities, and their disclosure could harm the interests of national security.

[64] Prosecution during the riots, Masimov and his deputy, Director of the Special Forces Service "A" Sadykulov, ordered their subordinates to leave the buildings of the KNB departments.

[65] An investigation into the fact that Masimov received an elite guest house and a land plot in the capital worth about 2.5 billion tenge from one of the business structures during his tenure as Prime Minister.

Former head of the National Security Committee Karim Massimov, through his wife's brother Mashurov Dilshat, is the owner of sixteen luxury houses in prestigious areas of different warm states in America.

[68] According to the Prosecutor General of Kazakhstan, the details of the criminal case and the court hearing were classified as containing state secrets, and information relating to foreign policy, counterintelligence and operational investigative activities, and their disclosure could harm the interests of national security.

[69] At the beginning of 2023, the Prosecutor General of Kazakhstan, Berik Asylov, named Masimov as the main organizer of mass riots in early January 2022.

Together with Masimov, Anvar Sadykulov, Daulet Ergozhin and Marat Osipov were convicted, who were sentenced to terms of 16, 15 and 3 years in prison, respectively, and were deprived of the rank of general and state awards.