Droysen's devotion to Prussia began in his boyhood, during the War of Liberation against Napoleonic rule. While professor of classical philology at Berlin (1835–40), he wrote on Alexander the Great and used the term Hellenism to describe the diffusion of Greek culture over the eastern Mediterranean and Middle East in the 4th–1st centuries BC.

After the revolution of 1848 Droysen became a member of the Frankfurt Parliament and secretary of its constitutional committee. After the Prussian king Frederick William IV refused the German imperial crown in 1849, Droysen, disappointed, retired from politics.

As professor of history at Kiel (1840–51), however, he collaborated in 1850 with Carl Samwer in writing a history of relations between Denmark and the duchies of Schleswig and Holstein from 1806, a work that affected the opinions of many Germans on the then-acute dispute with Denmark. He supported the rights of the duchies so prominently that in 1851, after Holstein passed to Denmark, he prudently left Kiel to teach at Jena, where he finished a biography (1851–52) of Graf Yorck von Wartenburg, Prussian general in the War of Liberation. He spent his remaining years on his great work, Geschichte der preussischen Politik, 14 vol. (1855–86; “History of Prussian Politics”). This history, unfinished at Droysen's death, ends at the year 1756.