He studied history at the Universities of Göttingen and Breslau, obtaining his habilitation at the University of Marburg in 1878. From 1880 he served as an archivist in Marburg, later working as a university lecturer in Marburg and Göttingen. From 1888 to 1901 he was associated with the Deutsches Historisches Institut Rom (German Historical Institute in Rome).
In 1901 he was appointed director of the Staatsarchiv Stettin (National Archives in Stettin), and in 1913 succeeded Georg Winter (1856–1912) as director of the State Archives of Magdeburg. Here he was co-author of Geschichte des Staatsarchiv Magdeburg (History of the State Archives of Magdeburg). He entered into retirement in 1923, relocating to Wernigerode during the following year.
Friedensburg's prime area of research involved the history of the Reformation. The following are a few of his better known publications:
- Der Reichstag zu Speier 1526, im Zusammenhang der politischen und kirchlichen Entwicklung Deutschlands im Reformationzeitalte, 1887 - The Diet of Speyer in 1526, in the context of political and religious development in Germany.
- Die chronik des Cerbonio Besozzi, 1548-1563, 1904 (edition) - Chronicles of Cerbonio Besozzi.
- Geschichte der Universität Wittenberg, 1917 - History of the University of Wittenberg.
- Die Protokolle der Kirchenvisitationen im Stift Merseburg von 1562 und 1578, 1931
- Kaiser Karl v. und papst Paul III. (1534-1549), 1932 - Emperor Charles V and Pope Paul III.
- Johannes Sleidanus, der geschichtsschreiber und die schicksalsmächte der reformationszeit, 1935 - book on historian Johannes Sleidanus (1506-1556).
- University of Magdeburg Biographies
- WorldCat Identities (list of publications)