As a playwright, he wrote: Pastor Ephraim Magnus (1917), which The Cambridge Guide to Theatre describes as a nihilistic, expressionist play "stuffed with perversities and sado-masochistic motifs"; Coronation of Richard III (1922; "equally lurid"); and a version of Medea (1926). Later works include the novel The Shoreless River (Fluss ohne Ufer, trilogy) and the drama Thomas Chatterton (1955; staged by Gustaf Gründgens in 1956). Erwin Piscator staged Jahnn's The Dusty Rainbow (Der staubige Regenbogen) in 1961.
Jahnns was also a music publisher, focusing on 17th-century organ music. He was a contemporary of organ-builder Rudolf von Beckerath.
He met Gottlieb Friedrich Harms at school (St. Pauli's Realschule) which he united in a "mystical wedding" in 1913, and they lived together between 1914 and 1918. They met Ellinor Philips in 1918. In 1919, Jahnn founded the community of Ugrino with a sculptor, Franz Buse. In 1926, Jahnn married Ellinor, and Harms married Sybille Philips, Ellinor's sister, in 1928.