He had a brother Matthew Rapf, known for producing the TV series Kojak and other television and film work.

In 1934, while majoring in English at Dartmouth, Rapf visited the Soviet Union as an exchange student, where he was impressed by the presentation of Communism he was shown.

Despite the danger to a Jew visiting Berlin at that time, he stopped there on the way home, an experience which he described in his autobiography as convincing him that Communism was the only thing capable of defeating Hitler, and greatly influenced his political views.

He entered the "family business" of film-making, and co-wrote screenplays for We Went to College (1936), They Gave Him a Gun (1937), and The Bad Man of Brimstone (1937).

In 1944 Rapf was recruited by Walt Disney to work on the screenplay for Song of the South (1946), from a treatment by screenwriting newcomer Dalton Reymond.

[8] According to journalist Neal Gabler, one of the reasons Disney hired Rapf was to temper what Disney feared would be Reymond's white Southern slant:Rapf was a minority, a Jew, and an outspoken left-winger, and he himself feared that the film would inevitably be Uncle Tomish.

"[10] In July 1946, Rapf was one of several people listed in a column by The Hollywood Reporter publisher William Wilkerson, identifying them as Communists and sympathizers.

"Billy's List" formed the basis for what became the Hollywood Blacklist,[11][12] and while Rapf was excused from testifying to the House Unamerican Activities Committee due to illness, being summoned effectively ended his career in the industry.

He did some further writing for film and television using "fronts" and pseudonyms, including Father Brown (aka, The Detective, 1954) starring Alec Guinness.