In 1941, her father was US ambassador to the United Kingdom, and he pulled strings to arrange for her a visa and a job as a reporter for Hearst's International News Service.

[3] In her account of the behind-the-scenes roles the three women played at the Yalta Conference, Catherine Grace Katz wrote that her father delegated to Mortimer the task of breaking off a distracting affair her father Harriman was having with Pamela Churchill, then Winston Churchill's young daughter-in-law.

Mortimer learned the Russian language during the three years she lived with her father there, and her wartime correspondence contains detailed descriptions of key Soviet leaders, and their wives.

[5] Historian Geoffrey Roberts wrote that, after first lady Eleanor Roosevelt, she was the second most well-known American woman in the Soviet Union.

Both Germany and the Soviet Union invaded Poland under the terms of Molotov–Ribbentrop Pact, and, in 1954, Mortimer was called as a witness to try to determine which nation had performed the mass summary execution.