In public life, he defended and asserted the rights of Americans to privacy and freedom from censorship, playing a significant role in challenging and overcoming the banning of certain works of literature (including James Joyce's Ulysses and Radclyffe Hall's The Well of Loneliness) and in asserting the right of media employees to organize labor unions.

[2] His father, Carl Ernst, had been born in Plzeň, Bohemia (in what is now the Czech Republic), and had worked as a peddler and shopkeeper; while his mother, Sarah Bernheim, was the daughter of German immigrants and had graduated from Hunter College.

[3] During the 1930s, Ernst played a significant role in challenging and relaxing existing censorship around the topics of sexual education and birth control, exonerating the sexual education manuals of Marie Stopes and Mary Ware Dennett,[1] as well as legally representing Margaret Sanger and Hannah Stone and defending Life magazine over a photographic essay related to the film The Birth of a Baby.

Ernst displayed considerable skill at harnessing the media to publicise and foreground his cases and initiatives, as well as his ability to educate a courtroom audience (and, frequently, its legal staff) on the topics in question.

[4] In 1933, on behalf of Random House, he successfully defended James Joyce's novel Ulysses against obscenity charges in the case of United States v. One Book Called Ulysses, leading to the book's publication in the U.S.[5][1] He won similar cases on behalf of Radclyffe Hall's The Well of Loneliness and Arthur Schnitzler's Casanova's Homecoming.

[3] By the early 1940s, Ernst was leaving the pursuit of individual cases behind in favor of committee work, legal education and liaison with state representatives.

White, Groucho Marx, Michael Foot, Compton Mackenzie, Al Capp, Charles Addams, Grandma Moses, Heywood Broun, and Margaret Bourke-White.