Dahn studied law and philosophy in Munich and Berlin (1849–53) and taught jurisprudence at the universities of Munich, Würzburg, Königsberg, and Breslau, where he was appointed rector in 1895.

Dahn's most substantial historical works are Die Könige der Germanen, 11 vol. (1861–1907; “The Kings of the Germanic People”); Die Urgeschichte der germanischen und romanischen Völker, 4 vol. (1881–90; “The History of the Origins of the Germanic and Latin Nations”); and Deutsche Geschichte von der Urzeit bis 843 (1883–88; “German History from the Beginning to 843”). His voluminous poetry, although consisting in large part of ballads, is best known for his verse epics on subjects of early German history: Harald und Theano (1854–55) and Die Amalungen (written in 1857–58, published in 1876; “The Amalings”). The historical novel, however, is the genre in which Dahn was most successful. He won great acclaim for Ein Kampf um Rom, 4 vol. (1876–78; “A Struggle for Rome”), in which he reconstructed the decline and fall of the Ostrogothic empire in Italy.