Hays served as the general counsel of the American Civil Liberties Union and was involved in the Scopes trial (along with Clarence Darrow), the Sacco and Vanzetti trial, and the Scottsboro trial. In 1933, Hays went to Germany to defend the communists accused of setting fire to the Reichstag. However, since he was a Jew he was not allowed to plead his case directly, but only through a German lawyer.