After the German invasion, Kéthly left Budapest and lived in the country with false papers under an assumed identity.

In June 1950, Kéthly, together with several other members of the Social Democratic Party, was arrested by the Communists, who had in the meantime gained control of Hungary.

In January 1954, after more than three years in prison, she was charged with spying and activities directed against the state and sentenced to life imprisonment.

On 3 November her Party nominated Kéthly for a ministerial position in the new coalition government of Imre Nagy but at dawn the following day, 4 November 1956, the Soviet Union invaded Hungary and she was advised to fly to New York City and appeal to the U.N. General Assembly on behalf of Hungary.

In 1962 the Hungarian Supreme Court reviewed Kéthly's 1954 pardon and, in absentia, imposed on her a three-year prison sentence for anti-state activities.