Ludwig was trained in law but at 25 began writing plays and poems. After serving as foreign correspondent for a German newspaper during World War I, he wrote a novel (Diana, originally published as two works, 1918–19; Eng. trans., 1929). In 1920 he published a biography of J.W. von Goethe, which established him as a writer in the “new school” of biography that emphasized the personality of the subject.

Ludwig's work has elicited a mixed response because his biographies combine fiction with fact. Many of his biographies have appeared in English translation: Napoleon (1927); Bismarck (1927); William Hohenzollern (1927); Goethe (1928); The Son of Man (1928), a highly controversial biography of Christ; Lincoln (1929); Hindenburg (1935); Cleopatra: The Story of a Queen (1937); Roosevelt: A Study in Fortune and Power (1938); Three Portraits: Hitler, Mussolini, Stalin (1940); and Beethoven (1943). Othello (1947) is an imaginative retelling of William Shakespeare's tragedy.