Werth's family fled to the United Kingdom in the wake of the Russian Revolution.
Werth wrote about France in the prewar period and about Russia in World War II, especially the Battle of Stalingrad and the Siege of Leningrad. He spoke and wrote both Russian and English at the native level. His best-known work is Russia at War 1941 to 1945, (London, 1964) a behind-the-scenes look at life in the wartime Soviet Union; he spent the war there as the BBC's correspondent, and had unrivalled access due to the combination of his BBC press credentials and his ability to function as a native Russian.
Werth was among a group of journalists to visit the Majdanek concentration and extermination camp after it had been discovered by the advancing Red Army. He filed a report on the atrocities, but the BBC initially refused to broadcast it, believing that it was too incredible to be true and suspecting a Soviet propaganda stunt.
He was the Moscow correspondent for the Guardian newspaper from 1946 to 1949. He was one of the first outsiders to be allowed into Stalingrad after the battle. Other works include: France 1940-1955: the de Gaulle Revolution; Moscow 41; The Last Days of Paris: a Journalist's Diary; Leningrad; The Year of Stalingrad; and Musical Uproar in Moscow.
- France in Ferment. London: Jerrolds, 1934.
- The Destiny of France. London: Hamish Hamilton, 1937. Published in USA as Which Way France.
- France And Munich Before And After The Surrender. London: Hamish Hamilton, 1939.
- The Last Days of Paris: A Journalist's Diary. London: Hamish Hamilton, 1940.
- Moscow '41. London: Hamish Hamilton, 1942. Published in USA as Moscow War Diary.
- The Twilight of France, 1933 - 1940: A Journalist's Chronicle. London: Hamish Hamilton, 1942.
- Leningrad. London: Hamish Hamilton, 1944.
- The Year of Stalingrad: An Historical Record and a Study of Russian Mentality, Methods and Policies. London: Hamish Hamilton, 1946.
- Musical Uproar in Moscow. London: Turnstile Press, 1947.
- France, 1940-1955. London: Robert Hale, 1956.
- The Strange History of Pierre Mende's-France and the Great Conflict over French North Africa. London: Barrie Books, 1957. Published in USA as Lost Statesman: The Strange Story of Pierre Mendes-France.
- America in Doubt. London: Robert Hale, 1959.
- The DeGaulle Revolution. London: Robert Hale, 1960.
- The Khrushchev Phase: The Soviet Union Enters the "Decisive" Sixties. London: Robert Hale, 1961. Published in USA as Russia Under Khrushchev.
- Russia At War, 1941-1945. London: Barrie & Rockliff, 1964.
- DeGaulle: A Political Biography. London: Simon and Schuster, 1965.
- Russia: Hopes and Fears. London: Barrie & Rockliff, 1969.
- Russia: The Post-War Years. London: Robert Hale, 1971.