Droysen was born at Treptow in Pomerania. His father, Johann Christoph Droysen, was an army chaplain who had been present at the celebrated siege of Kolberg in 1806–1807. As a child, Droysen witnessed some of the military operations during the War of Liberation, his father by then being pastor at Greifenhagen, in the immediate neighbourhood of Stettin, which was held by the French for most of 1813. These youthful impressions laid the foundation of his ardent attachment to the Kingdom of Prussia. He was educated at the gymnasium of Stettin and at the University of Berlin; in 1829 he became a master at the Graues Kloster, one of the oldest schools in Berlin; besides his work there he gave lectures at the Friedrich Wilhelm University, from 1833 as Privatdozent, and from 1835 as professor, without a salary.
During these years Droysen studied classical antiquity; he published a translation of Aeschylus and a paraphrase of Aristophanes, but the work by which he made himself known as a historian was his Geschichte Alexanders des Grossen (Berlin, 1833,– and other editions), a book which long remained probably the best work on the subject. It was in some ways the herald of a new school of German historical thought, for it shows that idealization of power and success which he had learnt from the teaching of Hegel. It was followed by other volumes dealing with the successors of Alexander the Great, published under the title of Geschichte des Hellenismus (Hamburg, 1836–1843). A new and revised edition of the whole work was published in 1885; it has been translated into French, but not into English.
In 1840 Droysen was appointed professor of history at Kiel. He was at once attracted to the political movement for the defence of the rights of the Elbe duchies, of which Kiel was the centre. Like his predecessor FC Dahlmann, he placed his historical learning at the service of the estates of Schleswig-Holstein and composed the address of 1844, in which the estates protested against the claim of King Christian VIII of Denmark to alter the law of succession in the duchies. In 1848 he was elected a member of the Frankfurt parliament, and acted as secretary to the committee for drawing up the constitution. He was a determined supporter of Prussian ascendancy, and was one of the first members to retire after King Frederick William IV of Prussia refused the imperial crown in 1849. During the next two years Droysen continued to support the cause of the duchies, and in 1850, with Carl Samwer, he published a history of the dealings of Denmark with Schleswig-Holstein, Die Herzogthümer Schleswig-Holstein und das Königreich Dänemark seit dem Jahre 1800 (Hamburg, 1850). A translation was published in London in the same year under the title The Policy of Denmark towards the Duchies of Schleswig-Holstein. The work was one of great political importance, and had much to do with the formation of German public opinion on the rights of the duchies in their struggle with Denmark.
After 1851 it was impossible for Droysen to remain at Kiel, and he was appointed to a professorship at Jena; in 1859 he was called to Berlin, where he remained till his death. In his later years he was almost entirely occupied with Prussian history. In 1851 he brought out a life of Count Yorck von Wartenburg (Berlin, 1851–1852), generally considered one of the best biographies in the German language, and then began his great work on the Geschichte der preussischen Politik (Berlin, 1855–1886). Seven volumes were published, the last not till after his death. It forms a complete history of the growth of the Prussian monarchy down to the year 1756. This, like all Droysen's work, shows a strongly marked individuality, and a great power of tracing the manner in which important dynamic forces worked themselves out in history. It was his characteristic quality of comprehensiveness that also gave him so much influence as a teacher.
Droysen was twice married, and died in Berlin. His eldest son, Gustav, was the author of several well-known historical works, namely, Gustav Adolf (Leipzig, 1869–1870); Herzog Bernhard von Weimar (Leipzig, 1885); an admirable Historischer Handatlas (Leipzig, 1885), and several writings on various events of the Thirty Years' War. Another son, Hans Droysen, was the author of some works on Greek history and antiquities.
See M. Duncker, Johann Gustav Droysen, ein Nachruf (Berlin, 1885); and Dahlmann-Waitz, Quellenkunde der deutschen Geschichte (Leipzig, 1906).