English essayist and critic, best known for his Confessions of an English Opium-Eater. De Quincey's biography of Samuel Taylor Coleridge appeared in the eighth edition of the Encyclopædia Britannica (see the Britannica Classic: Samuel Taylor Coleridge).

As a child De Quincey was alienated from his solid, prosperous mercantile family by his sensitivity and precocity. At the age of 17 he ran away to Wales and then lived incognito in London (1802–03). There he formed a friendship with a young prostitute named Ann, who made a lasting impression on him. Reconciled to his family in 1803, he entered Worcester College, Oxford, where he conceived the ambition of becoming “the intellectual benefactor of mankind.” He became widely read in many subjects and eventually would write essays on such subjects as history, biography, economics, psychology, and German metaphysics. While still at college in 1804, he took his first opium to relieve the pain of facial neuralgia. By 1813 he had become “a regular and confirmed opium-eater” (i.e., an opium addict), keeping a decanter of laudanum (tincture of opium) by his elbow and steadily increasing the dose; he remained an addict for the rest of his life.

De Quincey was an early admirer of Lyrical Ballads, and in 1807 he became a close associate of its authors, William Wordsworth and Samuel Taylor Coleridge. He rented Wordsworth's former home, Dove Cottage at Grasmere, on and off from 1809 to 1833. In 1817 De Quincey married Margaret Simpson, who had already borne him a son. Though he wrote voluminously, he published almost nothing. His financial position as head of a large family went from bad to worse until the appearance of Confessions (1821) in London Magazine made him famous. It was reprinted as a book in 1822.

The avowed purpose of the first version of the Confessions is to warn the reader of the dangers of opium, and it combines the interest of a journalistic exposé of a social evil, told from an insider's point of view, with a somewhat contradictory picture of the subjective pleasures of drug addiction. The book begins with an autobiographical account of the author's addiction, describes in detail the euphoric and highly symbolic reveries that he experienced under the drug's influence, and recounts the horrible nightmares that continued use of the drug eventually produced. The highly poetic and imaginative prose of the Confessions makes it one of the enduring stylistic masterpieces of English literature.

In 1856 he seized the opportunity provided by the publication of his collected works to rewrite the book that had made him famous. He added some descriptions of opium-inspired dreams that had appeared about 1845 in Blackwood's Magazine under the title Suspiria de Profundis (“Sighs from the Depths”). But by this time he had lost most of the accounts he had kept of his early opium visions, so he expanded the rather short original version of the Confessions in other ways, adding much autobiographical material on his childhood and his experiences as a youth in London. His literary style in the revised version of the Confessions, however, tends to be difficult, involved, and even verbose.

Among De Quincey's other autobiographical writings, the so-called Lake Reminiscences (first printed in Tait's Magazine, 1834–40), which deeply offended Wordsworth and the other Lake poets, remains of great interest, although it is highly subjective, not without malice, and unreliable in matters of detail. As a literary critic De Quincey is best known for his essay “On the Knocking at the Gate in Macbeth” (first printed in the London Magazine, October 1823), a brilliant piece of psychological insight and a classic of Shakespearean criticism.

De Quincey became increasingly solitary and eccentric, especially after his wife's death in 1837, and he often retreated for long periods into opium dreams. Of the more than 14 volumes of his work, only the original Confessions is a definitive literary expression.