Lewis is best known for his sensational Gothic novel The Monk (1796), a work which introduced the violence characteristic of German Gothic to the tradition established in England by Horace Walpole. His musical drama, The Castle Spectre (1798), increased his popularity.
Lewis served as a relatively ineffective member of Parliament from 1796 to 1802. He also owned a considerable amount of property in the West Indies, inherited from his father; his experience there led him to campaign for the humane treatment of slaves in his posthumously published Journal of a West India Proprietor (1834).
Lewis met a great many famous and influential people: Walter Scott said he was
fonder of great people than he ought to have been, either as a man of talent or a man of fortune. He had always dukes and duchesses in his mouth, and was particularly fond of anyone who had a title.But his associations included not only the nobility, but a great many writers, including Scott himself. While living in Weimar in 1792, he met "M. de Goethe, the celebrated author of Werter"; and he visited Byron, Percy Bysshe Shelley, and Polidori at the Villa Diodati in the summer of 1816. While there, he read aloud to Byron from Goethe's Faust (Part I), providing a running translation. He and Byron also visited Mme de Staël and argued about the slave trade.