Martin James Monti, United States Army Air Forces pilot, convicted of treason for defecting to the Waffen SS in 1944. He was paroled in 1960.
Robert Henry Best, convicted of treason on April 16, 1948 and served a life sentence. (Which turned out to be 4 years. Died in 1952 in prison of a cerebral hemorrhage.)
Iva Toguri D'Aquino, who is frequently identified by the name "Tokyo Rose", convicted 1949. Subsequently, pardoned by President Gerald Ford. (She was fined $10,000 and given a 10-year prison sentence. Paroled after 6.)
Mildred Gillars, also known as "Axis Sally", convicted of treason on March 8, 1949; served 12 years of a 10- to 30-year prison sentence.
Tomoya Kawakita, sentenced to death for treason in 1952, but eventually released by President John F. Kennedy to be deported to Japan. (In 1953, Eisenhower commuted sentence to $10,000 fine and life imprisonment. Kennedy ordered him released in 1963 on condition that he leave the US and not return.)
Robert Hanssen, a high-ranking FBI agent who was a spy for Russia, is spending several life sentences in the pokey but wasn't convicted of treason.