Molnár published his first stories at the age of 19 and achieved his first great success with the play Az ördög (1907; The Devil). Although trained for a career in law, he became instead a journalist and during World War I was a war correspondent.

A number of Molnár's plays, including Liliom (1909; Liliom), A hattyú (1920; The Swan), and A vörös malom (1923; The Red Mill), were successfully played abroad, particularly in Austria, Germany, and the United States. Some of them were made into films, but these translations and adaptations often emphasized the verbal beauty and romantic plots of his works at the expense of their finely detailed characterizations and their often bitter cynicism and biting irony. Some of Molnár's short stories, especially those collected in Muzsika (1908; “Music”), are masterpieces; concise and moving, they look beneath the glittering facade of society life to the problems of the poor and the underdog. Among his many novels, however, only A Pál utcai fíuk (1907; The Paul Street Boys) achieved much success. Molnár depicted the victory of evil, of egoism, and of immorality, but these elements were offset by his light, amusing touch. He spent the last years of his life in the United States.