Kilimnik was born on 27 April 1970 at
Kryvyi Rih,
Ukraine,
Soviet Union. Fluent in Russian and Ukrainian before his service in the
Soviet Army, he became fluent in Swedish and English as a linguist at the Moscow Military Institute of the Ministry of Defense, which trained interpreters for the Soviet Union's Main Intelligence Directorate, known as the
GRU of the Soviet Union. He was a translator in the
Soviet Army and worked closely with its GRU.He took Russian citizenship after the
dissolution of the Soviet Union. He worked in Sweden as an interpreter for a Russian arms dealer.
Kilimnik worked for the
International Republican Institute (IRI) in Moscow from 1995 to early 2005, the IRI is an organisation which receives funding from the United States government to support democracy. According to
anonymous sources, when applying for his position with the IRI, he responded to the question about how he learned English by stating that the "Russian military intelligence" taught him and he became known among Moscow political operatives as "Kostya, the guy from the GRU". In 1997, he traveled to the United States using a Russian
diplomatic passport.
The New York Times reported two former IRI colleagues said Kilimnik was dismissed in April 2005 after the chief of Russian Federation's
Federal Security Service gave a speech discussing internal private meetings at the institute. Kilimnik was suspected of leaking details of an IRI meeting in Bratislava, Slovakia. Kilimnik himself told
The New York Times in April 2018 that he had been dismissed for having freelanced as an interpreter for Manafort, which was effectively confirmed by a spokesman for the IRI who said such an action ran counter to the organization's code of ethics. Nearly a year later in February 2019, an IRI representative declined to say whether Kilimnik leaving the organisation had any connection to Kilimnik's alleged links to Russian intelligence. In the Mueller Report, a former colleague is reported to have told the
FBI that Kilimnik was fired because of his strong links to Russian intelligence services.