Conquest was born on 15 July 1917 in Great Malvern, Worcestershire, to an American father (Robert Folger Wescott Conquest) and an English mother (Rosamund Alys Acworth Conquest). His father served in an American Ambulance Service unit with the French Army in World War I, and was awarded the Croix de Guerre, with Silver Star in 1916.
Conquest was educated at Winchester College, the University of Grenoble, and Magdalen College, Oxford, where he was an exhibitioner in modern history and took his bachelor's and master's degrees in Philosophy, Politics and Economics, and his doctorate in Soviet history. In 1937, after studying at the University of Grenoble, Conquest went up to Oxford, joining both the Carlton Club and, as an "open" member, the Communist Party of Great Britain. Fellow members included Denis Healey and Philip Toynbee.
In Lisbon on an American passport at the outbreak of the Second World War, he returned to England. As the Communist party in Britain denounced the Second World War in 1939 as imperialist and capitalist, Conquest broke with it and enlisted in the Oxfordshire and Buckinghamshire Light Infantry, serving with the regiment from 1939 to 1946. In 1942, he married Joan Watkins, with whom he had two sons. In 1943, he was posted to the School of Slavonic and East European Studies, which is today part of University College London, to study Bulgarian. In 1944, Conquest was posted to Bulgaria as a liaison officer to the Bulgarian forces fighting under Soviet command, attached to the Third Ukrainian Front, and then to the Allied Control Commission. There, he met Tatiana Mihailova, who later became his second wife. At the end of the war, he joined the Foreign Office, returning to the British Legation in Sofia where he remained as the press officer. In 1948 he and Tatiana left Bulgaria when he was recalled to London under a minor diplomatic cloud after he had helped smuggle two Bulgarians out of the country. Back in London, he divorced his first wife and married Tatiana. In 1951, Tatiana Conquest was diagnosed with schizophrenia. In 1962 the couple divorced.
In 1948 Conquest joined the Foreign Office's Information Research Department (IRD), a "propaganda counter-offensive" unit created by the Labour Attlee government in order to "collect and summarize reliable information about Soviet and communist misdoings, to disseminate it to friendly journalists, politicians, and trade unionists, and to support, financially and otherwise, anticommunist publications." The IRD was also engaged in manipulating public opinion.
Conquest at the IRD was remembered as a "brilliant, arrogant" figure who had 10 people reporting to him. He continued to work at the Foreign Office until 1956, becoming increasingly involved in the intellectual counter-offensive against communism.