He served in Sri Lanka,[1] Austria, and for a lengthy period in the Soviet Union, where his tenure saw some of the most significant events of the Cold War.

[2] A 2019 assessment described him as "arguably the most influential figure who ever advised U.S. presidents about policy toward the Soviet Union during the Cold War.

Gary Powers's ill-fated U-2 high-altitude spy flight took place during his tenure, as did the American exhibition and the famous "kitchen debate" with Richard Nixon.

During the Cuban Missile Crisis, Thompson served on Kennedy's ExComm (Executive Committee of the National Security Council) when the US received two messages from Khrushchev, one quite conciliatory and the other much more hawkish.

Thompson's belief was that Khrushchev would be willing to withdraw the Soviet missiles from Cuba if he could portray the avoidance of a US invasion of the island as a strategic success.

Thompson was a pivotal participant in the formulation of Lyndon Johnson administration's non-proliferation policy on nuclear weapons and was instrumental in beginning the Strategic Arms Limitation Talks process.

[4] His daughters have written a biography published in March 2018 by Johns Hopkins University Press: "The Kremlinologist: Llewellyn E Thompson, America's Man in Cold War Moscow" (ISBN 978-1421424095).