He is a senator at the Federation Council (Russian parliament's upper house) and chairs its Foreign Affairs Committee.

After graduating from the Moscow State Institute of International Relations, Kosachev was a diplomat at the Russian Embassy in Sweden.

[citation needed] From May 1998 Kosachev was first an adviser, then an assistant for international affairs to the prime minister (under Sergey Kiriyenko, Yevgeny Primakov, and Sergei Stepashin).

[4] In 2003, Kosachev defended his Ph.D. thesis on "The concept of development of international law in the fight against nuclear terrorism" at the Diplomatic Academy of the Ministry of Foreign Affairs.

On 5 March 2012, Vladimir Putin appointed Kosachev head of Rossotrudnichestvo, the federal agency for the Commonwealth of Independent States, Compatriots Living Abroad and International Humanitarian Cooperation and the Russian president's special envoy for relations with CIS member-states.

On 21 September 2015, Kosachev was named representative of the Government of the Mari El Republic in the Russian Federation Council.

[10] In September 2015 Kosachev was included in the sanctions list of individuals and entities adopted the National Security and Defense Council of Ukraine.

[15] In June 2010, Kosachev wrote an article for the Echo of Moscow website proposing that the Russian Federation devise set of principles or a historical doctrine whereby it could disclaim any financial, legal, political or moral responsibility for USSR's actions in former territories while still acting as a successor state.

[16][17] In November 2011, he expressed the view that the European Monetary Union (euro zone) leave Greece, Hungary, Portugal, Ireland, Spain, Italy and Latvia, because these countries have a huge government debt.

[citation needed] In February 2017, following the resignation of Michael T. Flynn as national security adviser, Kosachev opined that Russophobia was pandemic in Donald Trump's administration.

[20] In September 2022, Kosachev warned that after the annexation referendums in Russian-occupied Ukraine, "protecting people in this region will not be our right, but our duty.