In 1818, weary of his somewhat restless and unsettled life, he returned to the practice of the Catholic faith and withdrew to the monastery of Dülmen, where he lived for some years in strict seclusion.

When she died, he prepared an index of the visions and revelations from her journal, The Dolorous Passion of Our Lord Jesus Christ (published 1833).

One of these visions made known by Brentano later resulted in the identification of the putative House of the Virgin Mary in Ephesus by Abbé Julien Gouyet, a French priest, during 1881.

However, some posthumous investigations in 1923 and 1928 made it uncertain how much of the books he attributed to Emmerich were actually his own creation and the works were discarded for her beatification process.

Brentano assisted Ludwig Achim von Arnim, his brother-in-law, in the collection of folk-songs forming Des Knaben Wunderhorn (1805–1808),[4] which Gustav Mahler drew upon for his song cycle.

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.

Of his dramas the best are Ponce de Leon (1804), Victoria und ihre Geschwister (Berlin, 1817) and Die Grundung Prags (Pesth, 1815).

A cycle of thirteen songs, based on Brentano's poems, is noted in Chapter XXI as one of the composer protagonist's most significant early works.