In New York Fles found temporary employment as a violinist, painting apartments, selling vacuum cleaners and working for publishers. In 1933 he established a literary agency in Manhattan, New York. Initially many of his clients were German refugees and other foreign authors. He organized evenings for these authors in New York, in order to get them acquainted with the American book market. From the 1940s onwards, however, most of his clientele was from the United States. Although he had no children of his own, Barthold Fles wrote two juvenile books. He also wrote introductions to compilations and many articles and translated several books from German to English. Among the translations was another children's book, Bambi's Children by Felix Salten. His non-fictional writings and his translations received considerable praise, except for his book on Germany. In 1986, at the age of 84, Fles closed his agency. Subsequently he returned to his native Netherlands, where he spent his last three years in Laren's Rosa Spier home for retired artists. At Rosa Spier he was approached by Madeleine Rietra, a Dutch expert on German literature, who posthumously published his letter exchange with clients Joseph Roth (bookchapter in 1991) and Heinrich Mann (book in 1993), along with commentaries and biographical notes.
He had no children of his own.