After 1947, Willetts joined the Foreign Office in the Moscow embassy.
[2] Willetts joined St Antony's College, Oxford in 1960.
Willetts was professor of Russian history at Oxford University.
He was director of the Russian and East European Centre at St Antony's College, where his colleagues included the noted Russian scholars Max Hayward, Harry Shukman and William Deakin.
In 1957 Willetts married Halina Szenbaum, a Polish Jew, in London, England.