In 1879 he qualified as a professor at the University of Vienna. Jellinek was later visiting professor of legal philosophy in Vienna, in 1881 he was named a member of the commission for state exams and one year later he published his seminal work, The Theory of the Unifications of States (1882). In 1883 he was given the prestigious title of Professor of Public Law at the University of Vienna. In 1889 he was duly given a professorship in Basel and left the academic service of Austria-Hungary. From 1891 he was Ordinarius for general Public Law and International Law at the University of Heidelberg. In 1900 he compiled his main work, the General Theory of the State.
He was married to Camilla Jellinek, née Wertheim (1860–1940), who was persuaded to join the Women's Movement by Marianne Weber in 1900 and became famous there especially for her work with providing women with legal aid and the production of draft reforms of the criminal law. The couple had six children, born between 1884 and 1896, of which just four survived childhood: son Walter, and daughter Dora, who was deported to the Theresienstadt concentration camp by the Nazis and youngest son, Otto, who died in 1943 as a result of abuse at the hands of the Gestapo.
Jellinek is best known for his essay The Declaration of the Rights of Man and the Citizen (1895), which argues for a universal theory of rights, as opposed to the culturally and nationally specific arguments then in vogue (particularly that of Émile Boutmy). Jellinek argued that the French Revolution, which was the focal point of 19th-century political theory, should not be thought of as arising from a purely French tradition (namely the tradition stemming from Jean-Jacques Rousseau) but as a close analogue of revolutionary movements and ideas in England and the United States.
Jellinek followed this in 1879 with his Habilitation at the University of Vienna. Jellinek then became a lecturer in the philosophy of law in Vienna, and in 1881 he was appointed a member of the State Examination Commission. In the following year, Jellinek published one of his seminal works titled 'The doctrine of Federated States' (1882). In 1883 he was appointed extraordinary professor of Constitutional Law in Vienna. In 1889 he became a full professor in Basel and retired from his academic position in Austria-Hungary. In 1891 Jellinek became a Professor of law at the University of Heidelberg and wrote, in 1900, his magnum opus, the General Theory of State. In 1907 he became the first Jewish Rector of Heidelberg University.