Massimov served as Deputy Prime Minister from 19 January 2006 to 9 January 2007 and as Minister of Economy and Budget Planning, Minister of Transport and Communications in 2001. President Nazarbayev nominated Massimov to succeed Daniyal Akhmetov as Prime Minister on 9 January 2007. The Nur Otan party endorsed Massimov's candidacy and Parliament confirmed the nomination on 10 January.
On 24 September 2012, Massimov's premiership ended when President Nazarbayev moved him to the position of chief of staff of the presidential office; according to Reuters, the move was designed to readjust the power balance between various factions within the government.
Born in Tselinograd (now Astana) in Kazakhstan, Karim Massimov graduated from People's Friendship University of Russia, Moscow, and then studied in Beijing and at Wuhan University in Hubei, China.
Karim Massimov is considered a China expert. China has become an important strategic partner for Kazakhstan in recent years and the two countries are working closely together to develop Kazakhstan's energy resources. Massimov visited Beijing, China with Finance Minister Natalya Korzhova, Transport and Communications Minister Serik Akhmetov, and Energy and Mineral Resources Minister Baktykozha Izmukhambetov from 16–17 November 2006. Massimov co-chaired the third meeting of the China-Kazakhstan Cooperation Committee with Chinese Vice Premier Wu Yi. Several accords between agencies of the two governments were signed. Massimov later met with Chinese Prime Minister Wen Jiabao. Meanwhile, Kazakh President Nursultan Nazarbayev met with Liu Qi, secretary of the Beijing Party Committee, in Astana. Secretary Liu said was "very nice for me to visit your beautiful country at Otan's invitation. The goal of the visit is to deepen cooperation and mutual understanding between our countries."
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 and Israeli Vice Premier Shimon Peres announced from Jerusalem on 29 October 2006 that the state-owned National Innovation Fund of Kazakhstan would begin investing in the Peace Valley project and other projects in the Middle East. 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. Kazakhstan intends to found political and economic ties with Israel and its neighbors." Massimov mentioned the Dead Sea canal and expressed desire to create a free trade zone. Vice Premier Peres and Massimov agreed to establish an agriculture-school in each country. The NIF has given USD $10 million to Israeli VC fund Vertex.
Massimov also met with Israeli Prime Minister Ehud Olmert, who praised Kazakhstan for showing a "beautiful face of Islam. Contemporary, ever-developing Kazakhstan is a perfect example of both economic development and interethnic accord that should be followed by more Muslim states."
The Middle East Times reported on 7 November 2006 that earlier that week Massimov noted interest in the infrastructure corridor project between Turkey and Israel and buying the oil refinery in Haifa, Israel which the Israeli Government plans to privatize in 2007.
The Senate voted on 2 November 2006 to change the deadline for registering real estate and property, originally set at 30 December, to the new date of 1 April 2007, after President Nazarbayev and Massimov called for an extension. The bill, if signed by Nazarbayev, is expected to generate an additional $30 million in annual taxes.
President Nazarbayev nominated Massimov to succeed Daniyal Akhmetov as Prime Minister on 9 January 2007. Akhmetov resigned on 8 January without explanation. Analysts attributed Akhmetov's political downfall to the President's criticism of his administrative oversight of the economy. The Parliament voted overwhelmingly in favor of the nomination on 10 January with 37 out of 39 Senators and 66 of 77 Assemblymen from the Majilis voting for Massimov. Akhmetov became the Defense Minister, replacing Mukhtar Altynbayev.
Massimov is fluent in Kazakh, Russian, Chinese, English, and Arabic. At the same time, he is thought to be well connected within the Kremlin.
Massimov is married, has three kids. His hobbies are books, Muay Thai, skiing, rock climbing and golf.
Massimov was the President of Federation of Amateur Muaythai of Asia (FAMA) in 2010. The FAMA and the Continental Federation of International Federation of Muaythai Amateurs (IFMA) in Asia is the first Continental Federation since 1991, supporting the work and efforts of the IFMA (). He then was nominated and went on to assume the position of Vice President of the IFMA, President of Muaythai federation in Kazakhstan and Vice-President of World Muaythai Council (WMC) ().
In 2012, Massimov was unanimously re-elected for another four-year term to head the Asian federation, which is recognised by the Olympic Council of Asia (). He showed his support for the sport in an interview in which he stated that the sport "brings together athletes from across the world to train and compete with honour and in the spirit of cultural exchange and understanding."