McLaughlin's magnum opus "A Constitutional History of the United States" (1935) won the 1936 Pulitzer Prize for History. Written for the average reader, the purpose is "to present briefly and clearly the constitutional history of the United States during nearly two centuries", not giving a history of constitutional law as announced by the courts, but of the development of constitutional principles in relation to political and social conditions and forces outside of the courtroom. "The most significant and conclusive constitutional decision was not rendered by a court of law but delivered at the famous meeting of General Grant and General Lee at Appomattox."