His sister was Bettina von Arnim, Goethe's correspondent. He studied in Halle and Jena, afterwards residing at Heidelberg, Vienna and Berlin. He was close to Wieland, Herder, Goethe, Friedrich Schlegel, Fichte and Tieck.
In 1801, he moved to Göttingen, and became a friend of Achim von Arnim. In 1804, he moved to Heidelberg and worked with Arnim on Zeitungen für Einsiedler and Des Knaben Wunderhorn. In the years between 1808 and 1818, he lived mostly in Berlin, and from 1819 to 1824 in Dülmen, Westphalia.
In 1818, weary of his somewhat restless and unsettled life, he joined the Roman Catholic Church and withdrew to the monastery of Dulmen, where he lived for some years in strict seclusion. The latter part of his life he spent in Regensburg, Frankfurt and Munich, actively engaged in Catholic propaganda. He died at Aschaffenburg.
Brentano, whose early writings were published under the pseudonym Maria, belonged to the Heidelberg group of German romantic writers, and his works are marked by excess of fantastic imagery and by abrupt, bizarre modes of expression. His first published writings were Satiren und poetische Spiele (1800), and a romance Godwi (1801-1802); of his dramas the best are Ponce de Leon (1804), Victoria (1817) and Die Grundung Prags (1815).
On the whole his finest work is the collection of Romanzen vom Rosenkranz (published posthumously in 1852); his short stories, and more especially the charming Geschichte vom braven Kasperl und dem schönen Annerl (1838), which has been translated into English, were very popular. Brentano also assisted Ludwig Achim von Arnim, his brother-in-law, in the collection of folk-songs forming Des Knaben Wunderhorn (1806-1808).
Brentano's collected works, edited by his brother Christian, appeared at Frankfurt in 9 vols. (1851-1855). Selections have been edited by JB Diel (1873), M Koch (1892), and J Dohmke (1893). See JB Diel and W Kreiten, Klemens Brentano (2 vols, 1877-1878), the introduction to Koch's edition, and R Steig, A. von Arnim und K. Brentano (1894).