In March 1851 Grünhagen passed his secondary school teaching exams ("Oberlehrerexamen") in Breslau. At Easter 1853 he began work as a teaching assistant at the Friedrichs-Gymnasium (secondary school) in the city, becoming a full teacher there a few months later. Following his Habilitation from the University of Breslau in 1855, he took a university position as a History teacher ("...als Privatdozent"). In 1860 he published "Henrucus Pauper", described as a book of accounts for the city of Breslau between 1299 and 1358, using city records and other historical sources: the work comprised the third volume in the "Codex diplomaticus Silesiae" series. It was not until after he had resigned from the city's schools service, however, that on 11 March 1862 he succeeded Wilhelm Wattenbach as head of the Silesian Regional Archive (renamed "Silesian state/national archive" in 1867). For nearly forty years, until 1 April 1901, he remained in the post. During this period he received the titles of "Archivrat" in 1873 and "Geheim Archivrat" (very loosely, "Privy Archives Secretary") in 1885. The prestigious titles dated back to the late medieval period, but the core responsibilities of the head of the archives department appear to have remained broadly constant. He combined this and other positions with the editorship of the journal of the Silesian Historical Association ("Verein für Geschichte und Alterthum Schlesiens") between 1863 and 1905. Through his long tenure in these positions and as a result of the impressive quantity and detail of his output on Silesian history, Grünhagen became highly influential in his chosen specialist field, as well as with the Historical Association and the historian community more broadly. On 18 December 1866 he accepted an extraordinary professorship in History from the University of Breslau, where he continued to teach until 1911.
